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HARPER'S SCIENTIFIC MEMOIRS

EDITED BY

J. S. AMES, Ph.D.

PROFESSOR OF PHYSICS IN JOHNS HOPKINS UNIVERSITY

III.

RONTGEX RAYS

Digitized by the Internet Archive

in 2011 with funding from

Research Library, The Getty Research Institute

http://www.archive.org/details/rontgenraysmemoirOOront

R O NTGEN RAYS

MEMOIRS BY ROXTGEK, STOKES
AXD J. J. THOMSON

TRANSLATED AND EDITED BY

GEORGE F. BARKER, LL.D.

PROFESSOR OF PHYSICS IN THE UNIVERSITY OF PENNSYLVANIA

NEW YORK AND LONDON

HARPER & BROTHERS PUBLISHERS

1899

HARPER'S SCIENTIFIC MEMOIRS.

EDITED BY

J. S. AMES, Ph.D.,

PROFK880B OF PHYSIOS IN JOHNS UOPKIN8 UNIVEUSITY.

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PREFACE

The new kind of radiation known as X-rays, or Eontgen
rays, from the name of their discoverer, were first observed and
studied by Professor W. C. Rontgen, of the University of Wilrz-
burg, in 1895, and the announcement of their discovery was
made in a paper which appeared that year, and which is re-
printed in this volume. As was noticed later these radiations
had been previously detected and some of their properties
noted by other observers, notably Professor Lenard ; but it is
to Rontgen that we owe the first systematic study of the meth-
ods of production and of the remarkable properties of these
rays. Nearly all the general properties, both positive and neg-
ative, were investigated by Rontgen and carefully stated.
These results are contained in the first three pages of this
volume.

The most important experiments, however, and those which
have led to the most important conclusions, were made by Pro-
fessor J. J. Thomson, of Cambridge. They proved the fact
that a dielectric traversed by these radiations became a con-
ductor, or, in other words, was ionized. This discovery in the
hands of Professor Thomson and his students has led to a
series of most interesting and important researches, all bearing
upon the intimate connection between matter and electricity.

Many hypotheses have been advanced to accouut for the pe-
culiar properties of the X-rays. Rontgen himself at first was
favorably inclined to the idea that they were waves due to lon-
gitudinal vibrations in the ether, but later he was convinced
that they were essentially identical with light waves — that is,

PREFACE

with transverse waves in the ether. There were grave obsta-
cles, from many stand-points, to either of these theories, and
the first suggestion which seemed to offer a satisfactory expLa-
nation of all the properties of the rays came when, instead of
waves, the idea of pulses in the ether was introduced. This
idea in its simplicity is that the cathode rays being negative-
ly charged and travelling with great velocity, give rise to in-
tensely sudden disturbances in the ether when their motions
are stopped by reaching a solid obstacle. These disturbances
are of the nature of irregular pulses, and their properties are
quite different from those of regular trains of waves.

This idea of accounting for Rontgen rays by the theory of
pulses occurred almost simultaneously to Sir George Gabriel
Stokes, to Professor J. J. Thomson, and to Professor Lehmann,
of Karlsruhe. Stokes's paper, in which he explains his theory,
is reproduced in full in this volume, as are also the essential
portions of Professor Thomson's article.

GENERAL CONTEIS^TS

PAGE

Preface v

A New Kind of Rays. First Communication. By W. C. Rontgen. ... 3

Second Communication. By W. C. Rontgen. . 13
Furtlier Observations on the Properties of the X-Rays. Bj^ W. C.

Rontgen 21

Biographical Sketch of Rontgen 40

On the Nature of the Rontgen Rays. ( The Wilde Lecture. )

By Sir G. G. Stokes, Bart 43

Biographical Sketch of Stokes 66

A Theory of the Connection between Cathode and Rontgen Rays.

By J. J. Thomson 69

Biographical Sketch of Thomson 73

Bibliography 74

Index 75

ON A NEW KIND OF RAYS

TWO CO 21 M UNI CATIONS

Sitzungsheridite der Wurzburger PJiysikaUschen-^IediciniscJien Gesellschoft,
1895 — Wiedemann, Annalen der Physih iind Chemie, 64, 1898

CONTENTS

FraST COMMUNICATION

PAGE

Discovei^ of X-Rays 3

' ' Transparency " of Substances 4

Pi'operiies Investigated.

Fluorescence 6

PJwtograpMc Action 6

Refraction and Reflection 7

Variation of Intensity icith Distance 10

Magnetic Deflection 10

Point of Emission of Rays 11

Propagation in Straight Lines 11

Interference , 12

Polarization 12

Possible Explanations 12

Longitudinal Vibrations in the Ether 13

SECOND COMMUNICATION

Electrical Properties of X-Rays 13

Discharge of Electnfied Bodies 14

Action on a Dielectric 14

Duration of Effect on Air and Otiier Gases 15

Centre of Emission of the Rays 17

Eff^ect of Different Metals 18

ON A NEW KIND OF RAYS

T\'. C. ROXTGEN

FIRST CO MM UXI CATIO N

1. If the discharge of a fairly large iuduction-coil be made to
pass through a Hittorf vacuum-tube, or through a Lenard tube,
a Crookes tube, or other simiUir apparatus, which has been suf-
ficiently exhausted, the tube being covered with thin, black
card-board which fits it with tolerable closeness, and if the
whole apparatus be placed in a completely darkened room, there
is observed at each discharge a bright illumination of a pa-
per screen covered with barium platino-cyanide, placed in the
vicinity of the induction-coil, the fluorescence thus produced
being entirely independent of the fact whether the coated or
the plain surface is turned towards the discharge-tube. This
fluorescence is visible even when the paper screen is at a dis-
tance of two metres from the apparatus.

It is easy to prove that the cause of the fluorescence proceeds
from the discharge-apparatus, and not from any other point in
the conducting circuit.

2. The most striking feature of this phenomenon is the fact
that an active agent here passes through a black card-board en-
velope, which is opaque to the visible and the ultra-violet rays
of the sun or of the electric arc ; an agent, too, which has the
power of producing active fluorescence. Hence we may first
investigate the question whether other bodies also possess this
property.

We soon discover that all bodies are transnarent to this asrent,

MEMOIRS ON

though in very different degrees. I proceed to give a few ex-
amples : Paper is very transparent ; * behind a bound book of
about one thousand pages I saw the fluorescent screen light
up brightly, the printers' ink offering scarcely a noticeable
hinderance. In the same way the fluorescence appeared behind
a double pack of cards ; a single card held between the ap-
paratus and the screen being almost unnoticeable to the eye.
A single sheet of tin-foil is also scarcely perceptible ; it is only
after several layers have been placed over one another that
their shadow is distinctly seen on the screen. Thick blocks
of wood are also transparent, pine boards two or three centi-
metres thick absorbing only slightly. A plate of aluminium
about fifteen millimetres thick, though it enfeebled the action
seriously, did not cause the fluorescence to disappear entirely.
Sheets of hard rubber several centimetres thick still permit the
rays to pass through them.f Glass plates of equal thickness
behave quite differently, according as they contain lead (flint-
glass) or not ; the former are much less transparent than the
latter. If the hand be held between the discharge-tube and
the screen, the darker shadow of the bones is seen within the
slightly dark shadow-image of the hand itself. Water, carbon
disulphide, and various other liquids, when they are examined
in mica vessels, seem also to be transparent. That hydrogen is
to any considerable degree more transparent than air I have
not been able to discover. Behind plates of copper, silver,
lead, gold, and platinum the fluorescence may still be recog-
nized, though only if the thickness of the plates is not too
great. Platinum of a thickness of 0.2 millimetre is still trans-
parent ; the silver and copper plates may even be thicker.
Lead of a thickness of 1.5 millimetres is practically opaque ;
and on account of this property this metal is frequently most

* By "transparency" of a bod}^ I denote the relative brightness of a
fluorescent screen placed close behind the body, referred to tiie brightness
which the screen shows under the same circumstances, though without the
interposition of the body.

\ For brevity's sake I shall use the expression " rays"; and to distinguish
them from others of this name I shall call them " X-rays." (See p. 11.)

ROXTGEN RAYS

useful. A rod of wood with a square cross -section (20 x 20
millimetres), one of whose sides is painted white with lead paint,
behaves differently according as to how it is held between the
ajDparatus and the screen. It is almost entirely without action
when the X-rays pass through it parallel to the painted side ;
whereas the stick throws a dark shadow when the rays are
made to traverse it perpendicular to the painted side. In a
series similar to that of the metals themselves their salts can
be arranged with reference to their transparency, either in the
solid form or in solution.

3. The experimental results which have now been given,
as well as others, lead to the conclusion that the transpar-
ency of different substances, assumed to be of equal thick-
ness, is essentially conditioned upon their density : no other
property makes itself felt like this, certainly to so high a de-
gree.

The following experiments show, however, that the density
is not the only cause acting. I have examined, with reference
to their trans2:)arency, plates of glass, aluminium, calcite, and
quartz, of nearly the same thickness ; and while these sub-
stances are almost equal in density, yet it was quite evident
that the calcite was sensibly less transparent than the other
substances, which appeared almost exactly alike. Xo particu-
larly strong fluorescence (see p. 6 below) of calcite, especially
by comparison with glass, has been noticed.

4. All substances with increase in thickness become less
transparent. In order to find a possible relation between trans-
parency and thickness, I have made photographs (see p. 6 be-
low) in which portions of the photographic plate were covered
with layers of tin-foil, varying in the number of sheets super-
posed. Photometric measurements of these will be made when
I am in possession of a suitable photometer.

5. Sheets of platinum, lead, zinc, and aluminium were rolled
of such thickness that all appeared nearly equally transj^arent.
The following table contains the absolute thickness of these
sheets measured in millimetres, the relative thickness referred
to that of the platinum sheet, and their densities :

MEMOIRS ON

Thickness Rklative Thickness

Densiti

Pt 0.018 mm. 1

21.5

Pb 0.05 '' 3

11.3

ZnO.lO " 6

7.1

Al 3.5 '' 200

2.6

We may conclude from these values that different metals
possess transparencies which are by no means equal, even when
the product of thickness and density are the same. The trans-
parency increases much more rapidly than this product de-
creases.

6. The fluorescence of barium platino- cyanide is not the
only recognizable effect of the X-rays. It should be mentioned
that other bodies also fluoresce ; such, for instance, as the phos-
phorescent calcium compounds, then uranium glass, ordinary
glass, calcite, rock-salt, and so on.

Of special signiflcance in many respects is the fact that
photographic dry plates are sensitive to the X-rays. We are,
therefore, in a condition to determine more definitely many
phenomena, and so the more easily to avoid deception ; wher-
ever it has been possible, therefore, I have controlled, by means
of photography, every important observation which I have made
with the eye by means of the fluorescent screen.

In these experiments the property of the rays to pass almost
unhindered through thin sheets of wood, paper, and tin-foil is
most important. The photographic impressions can be ob-
tained in a non-darkened room with the j)hotograpliic plates
either in the holders or wrapped up in paper. On the other
hand, from this property it results as a consequence that un-
developed plates cannot be left for a long time in the neighbor-
hood of the discharge-tube, if they are protected merely by the
usual covering of pasteboard and paper.

It appears questionable, however, whether the chemical ac-
tion on the silver salts of the photographic plates is directly
caused by the X-rays. It is possible that this action proceeds
from the fluorescent light which, as noted above, is produced

RONTGEN RAYS

in the glass plate itself or perhaps in the layer of gelatin.
"Films " can be used just as well as glass plates.

I have not yet been able to prove experimentally that the
X-rays are able also to produce a heating action; yet we may
well assume that this effect is present, since the capability of
the X-rays to be transformed is proved by means of the ob-
served fluoresence phenomena. It is certain, therefore, that
all the X-rays which fall upon a substance do not leave it again
as such.

The retina of the eye is not sensitive to these rays. Even if
the eye is brought close to the discharge-tube, it observes noth-
ing, although, as experiment has proved, the media contained
in the eye must be sufficiently transparent to transmit the
rays.

7. After I had recognized the transparency of various sub-
stances of relatively considerable thickness, I hastened to see
how the X-rays behaved on passing through a prism, and to
find whether they were thereby deviated or not.

Experiments with water and with carbon disulphide enclosed
in mica prisms of about 30° refracting angle showed no devia-
tion, either with the fluorescent screen or on the photographic
plate. For purposes of comparison the deviation of rays of
ordinary light under the same conditions was observed; and it
was noted that in this case the deviated images fell on the
plate about 10 or 20 millimetres distant from the direct image.
By means of prisms made of hard rubber and of aluminium,
also of about 30° refracting angle, I have obtained images
on the photographic plate in which some small deviation may
perhaps be recognized. However, the fact is quite uncertain;
the deviation, if it does exist, being so small that in any case
the refractive index of the X-rays in the substances named
cannot be more than 1.05 at the most. With a fluorescent
screen I was also unable to observe any deviation.

Up to the present time experiments with prisms of denser
metals have given no definite results, owing to their feeble
transparency and the consequently diminished intensity of the
transmitted rays.

MEMOIRS ON

"Witli reference to the general conditions here involved on
the one hand, and on the other to the im2:)ortance of the ques-
tion whether the X-rays can be refracted or not on i:)assing
from one medium into another, it is most fortunate that this
subject may be investigated in still another way than with
the aid of prisms. Finely divided bodies in sufficiently thick
layers scatter the incident light and allow only a little of it
to pass, owing to reflection and refraction ; so that if powders
are as transparent to X-rays as the same substances are in mass
— equal amounts of material being presupposed — it follows at
once that neither refraction nor regular reflection takes place
to any sensible degree. Experiments w^ere tried with finely
powdered rock-salt, with fine electrolytic silver-powder, and
with zinc-dust, such as is used in chemical investigations. In
all these cases no difference was detected between the trans-
parency of the powder and that of the substance in mass,
either by observation with the fluorescent screen or with the
photographic plate.

From what has now been said it is obvious that the X-rays
cannot be concentrated by lenses ; neither a large lens of hard
rubber nor a glass lens having any influence upon them. The
shadow-picture of a round rod is darker in the middle than at
the edge ; while the image of a tube which is filled with a sub-
stance more transparent than its own material is lighter at the
middle than at the edge.

8. The question as to the reflection of the X-rays may be re-
garded as settled, by the experiments mentioned in the pre-
ceding paragraph, in favor of the view that no noticeable regu-
lar reflection of the rays takes place from any of tlie substances
examined. Other experiments, which I here omit, lead to the
same conclusion.

One observation in this connection should, liowever, be men-
tioned, as at first sight it seems to prove the opposite. I ex-
posed to the X-rays a photographic plate which was protected
from the light by black paper, and the glass side of which was
turned towards the discharge - tube giving the X-rays. The
sensitive film was covered, for the most part, with polished

RONTGEN RA.YS

plates of platinum, lead, zinc, and aluminium arranged in the
form of a star. On the developed negative it was seen plainly
that the darkening under the platinum, the lead, and particu-
larly the zinc, was stronger than under the other plates, the
aluminium having exerted no action at all. It appears, there-
fore, that these three metals reflect the rays. Since, however,
other explanations of the stronger darkening are conceivable,
in a second experiment, in order to be sure, I placed between
the sensitive film and the metal plates a piece of thin alumin-
ium-foil, which is opaque to ultra-violet rays, but is very trans-
parent to the X-rays. Since the same result substantially was
again obtained, the reflection of X-rays from the metals above
named is proved.

If we compare this fact with the observation already men-
tioned that powders are as transparent as coherent masses, and
with the further fact that bodies with rough surfaces behave
like polished bodies with reference to the passage of the X-rays,
as shown also in the last experiment, we are led to the con-
clusion already stated that regular reflection does not take
place, but that bodies behave towards the X-rays as turbid
media do towards light.

Since, moreover, I could detect no evidence of refraction of
these rays in passing from one medium into another, it would
seem that X-rays move with the same velocity in all substances ;
and, further, that this speed is the same in the medium which
is present everywhere in sj)ace and in which the particles of
matter are imbedded. These particles hinder the propagation
of the X-rays, the effect being greater, in general, the more
dense the substance concerned.

9. Accordingly it might be possible that the arrangement of
particles in the substance exercised an influence on its trans-
parency; that, for instance, a piece of calcite might be trans-
parent in different degrees for the same thickness, according as
it is traversed in the direction of the axis, or at right angles to
it. Experiments, however, on calcite and quartz gave a nega-
tive result.

10. It is well known tjuit Lenard came to the conclusion,

xMEMOIRS ON

from the results of his beautiful experiments on the transmis-
sion of the cathode rays of Hittorf through a thin sheet of
aluminium, that these rays are phenomena of the ether, and
that they diffuse themselves through all bodies. We can say
the same of our rays.

In his most recent research, Lenard has determined the ab-
sorptive power of different substances for the cathode rays, and,
among others, has measured it for air from atmospheric press-
ure to 4.10, 3.40, 3.10, referred to 1 centimetre, according to
the rarefaction of the gas contained in the discharge-apparatus.
Judging from the discharge - pressure as estimated from the
sparking distance, I have had to do in my experiments for the
most part with rarefactions of the same order of magnitude,
and only rarely with less or greater ones. I have succeeded in
comparing by means of the L. Weber photometer — I do not
possess a better one^ — the intensities, taken in atmospheric air,
of the fluorescence of my screen at two distances from the dis-
charge-apparatus— about 100 and 200 millimetres; and I have
found from three experiments, which agree very well with each
other, that the intensities vary inversely as the squares of the
distances of the screen from the discharge-apparatus. Accord-
ugly, air absorbs a far smaller fraction of the X-rays than of
le cathode rays. This result is in entire agreement with the
c ervation mentioned above, that it is still possible to detect
the fluorescent light at a distance of 2 metres from the dis-
charge-apparatus.

Other substances behave in general like air; they are more
transparent to X-rays than to cathode rays.

11. A further difference, and a most important one, between
the behavior of cathode rays and of X-rays lies in the fact
that I have not succeeded, in spite of many attempts, in ob-
taining a deflection of the X-rays by a magnet, even in very in-
tense fields.

The possibility of deflection by a magnet has, up to the pres-
ent time, served as a characteristic property of the cathode
rays; although it was observed by Hertz and Lenard that tliere
are different sorts of cathode rays, '' wliicli are distinguished

RONTGEN RAYS

from each other by their production of phosphoresceuce, by the
amount of their absorption, and by the extent of their deflec-
tion by a magnet/^ A considerable deflection, however, was
noted in all of the cases investigated by them ; so that I do not
think that this characteristic will be given up except for strin-
gent reasons.

12. According to experiments especially designed to test the
question, it is certain that the spot on the wall of the dis-
charge-tube which fluoresces the strongest is to be considered
as the main centre from which the X-rays radiate in all direc-
tions. The X-rays proceed from that spot where, according
to the data obtained by different investigators, the cathode
rays strike the glass wall. If the cathode rays within the dis-
charge-apparatus are deflected by means of a magnet, it is ob-
served that the X-rays proceed from another spot — namely,
from that which is the new terminus of the cathode rays.

For this reason, therefore, the X-rays, which it is impossible
to deflect, cannot be cathode rays simply transmitted or re-
flected without change by the glass wall. The greater density
of the gas outside of the discharge-tube certainly cannot ac-
count for the great difference in the deflection, according to
Lenard. .

I therefore reach the conclusion that the X-rays are not
identical with the cathode rays, but that they are produced by
the cathode rays at the glass wall of the discharge-apj^aratus.

13. This production does not take place in glass alone, but,
as I have been able to observe in an apparatus closed by a
plate of aluminium 2 millimetres thick, in this metal also.
Other substances are to be examined later.

14. The justification for calling by the name ^^ rays "the
agent which proceeds from the wall of the discharge-apparatus
I derive in part from the entirely regular formation of shad-
ows, which are seen when more or less transparent bodies are
brought between the apparatus and the fluorescent screen (or
the photographic plate).

I have observed, and in part photographed, many shadow-
pictures of this kind, the production of which has a particular

MEMOIRS ON

charm. I possess, for instance, photographs of the shadow of
the profile of a door which separates the rooms in which, on
one side, the discharge-apparatus was placed, on the other the
photographic plate ; the shadow of the bones of the liand ; the
shadow of a covered wire wrapped on a wooden sjoool ; of a set
of weights enclosed in a box ; of a galvanometer in which the
magnetic needle is entirely enclosed by metal; of a piece of
metal whose lack of homogeneity becomes noticeable by means
of the X-rays, etc.

Another conclusive proof of the rectilinear propagation of
the X-rays is a pin-hole photograph which I was able to make
of the discharge-apparatus while it was enveloped in bhick pa-
per; the picture. is weak but unmistakably correct.

15. I have tried in many ways to detect interference phe-
nomena of the X-rays ; but, unfortunately, without success,
perhaps only because of their feeble intensity.

16. Experiments have been begun, but are not yet finished,
to ascertain whether electrostatic forces affect the X-rays in
any way.

17. In considering the question what are the X-rays — which,
as we have seen, cannot be cathode rays — we may perhaps at
first be led to think of them as ultra-violet light, owing to their
active fluorescence and their chemical actions. But in so doing
we find ourselves opposed by the most weighty considerations.
If the X-rays are ultra-violet light, this light must have the
following properties:

(a) On passing from air into water, carbon disulphide, alu-
minium, rock-salt, glass, zinc, etc., it suffers no noticeable re-
fraction.

(b) By none of the bodies named can it be regularly reflected
to any appreciable extent.

(d) Its absorption is influenced by no other property of
substances so much as by their density.

That is to say, we must assume that these ultra-violet rays
behave entirely differently from the ultra -red, visible, and
ultra-violet rays which have been known up to this time.

ROXTGEX RAYS

I have been nnable to come to this conclusion, and so have
songht for another explanation.

There seems to exist some kind of relationship between the
new rays and light rays ; at least this is indicated by the for-
mation of shadows, the fluorescence and the chemical action
produced by them both. Xow, we have known for a long time
that there can be in the ether longitudinal vibrations besides
the transverse light-vibrations ; and, according to the views of
different physicists, these vibrations must exist. Their exist-
ence, it is true, has not been proved up to the present, and
consequently their properties have not been investigated by
experiment.

Ought not, therefore, the new rays to be ascribed to longitu-
dinal vibrations in the ether ?

I must confess that in the course of the investigation I have
become more and more confident of the correctness of this idea,
and so, therefore, permit myself to announce this conjecture,
although I am perfectly aware that the explanation given still
needs further confirmation.

WuRZBURG, Physikalisches Institut der Universitat.
December^ 1895.

SECOXD COMMUXICA TIOX

Since my work must be interrupted for several weeks, I take
the opportunity of presenting in the following paper some new
phenomena which I have observed.

18. It was known to me at the time of my first publication
that X-rays can discharge electrified bodies ; and I conjecture
that in Lenard's experiments it was the X-rays, and not the
cathode rays, which had passed unchanged through the alu-
minium window of his apparatus, which produced the action
described by him upon electrified bodies at a distance. I have,
however, delayed the publication of my experiments until I
could contribute results which are free from criticism.

These results can be obtained only when the observations are
made in a space which is protected completely, not only from

MEMOIRSON

the electrostatic forces proceeding from the vacuum-tnbe, from
the conducting wires, from the induction apparatus, etc., but is
also closed against air which comes from the neighborhood of
the discharge-apparatus.

To secure these conditions I had a chamber made of zinc
plates soldered together, which was large enough to contain
myself and the necessary apparatus, which could be closed air-
tight, and which was provided with an opening which could
be closed by a zinc door. The wall opposite the door was for
the most part covered with lead. At a place near the dis-
charge-apparatus, which was set up outside the case, the zinc
wall, together with the lining of sheet-lead, was cut out for a
width of 4 centimetres; and the opening was covered again
air-tight with a thin sheet of aluminium. The X-rays pene-
trated through this window into the observation space.

I observed the following phenomena :

(a) Electrified bodies in air, charged either positively or
negatively, are discharged if X-rays fall upon them ; and this
process goes on the more rapidly the more intense the rays are.
The intensity of the rays was estimated by their action on a
fluorescent screen or a photographic plate.

It is immaterial in general whether the electrified bodies are
conductors or insulators. Up to the present I have not found
any specific difference in the behavior of different bodies with
reference to the rate of discharge ; nor as to the behavior of
positive and negative electricity. Yet it is not impossible that
small differences may exist.

{!)) If the electrified conductor be surrounded not by air but
l)y a solid insulator, c. g. paraffin, the radiation has the same
action as would result from exposure of the insulating envelope
to a flame connected to the earth.

{c) If this insulating envelope be surrounded by a close-
fitting conductor which is connected to tlie earth, and which,
like tlic insulator, is transparent to X-rays, the radiation pro-
duces on the inner electrified conductor no action which can
be detected by my apparatus.

{(1) The observations noted under (a), (/>), (r) indicate that

RONTGEN RAYS

air through which X-rays have passed possesses the power of
discharging electrified bodies with which it conies in con-
tact.

(e) If this is really the case, and if, further, the air retains
this property for some time after it has been exposed to the
X-rays, then it must be possible to discharge electrified bodies
which have not been themselves exposed to the rays, by con-
ducting to them air which has thus been exposed.

We may convince ourselves in various ways that this con-
clusion is correct. One method of experiment, although per-
haps not the simplest, I shall describe.

I used a brass tube 3 centimetres wide and 45 centimetres
long; at a distance of some centimetres from one end a part
of the wall of the tube was cut away and replaced by a thin
aluminium plate ; at the other end, through an air-tight cap, a
brass ball fastened to a metal rod was introduced into the tube
in such a manner as to be insulated. Between the ball and the
closed end of the tube there was soldered a side-tube which
could be connected with an exhaust-apparatus; so that when
this is in action the brass ball is subjected to a stream of air
which on its way through the tube has passed by the alumin-
ium window. The distance from the window to the ball was
over 20 centimetres.

I arranged this tube inside the zinc chamber in such a posi-
tion that the X-rays could enter through the aluminium win-
dow of the tube perpendicular to its axis. The insulated ball
lay then in the shadow, out of the range of the action of these
rays. The tube and the zinc case were connected by a con-
ductor, the ball was joined to a Hankel electroscope.

It was now observed that a charge (either positive or nega-
tive) given to the ball was not influenced by the X-rays so
long as the air remained at rest in the tube, but that the
charge instantly decreased considerably if by exhaustion the
air which had been subjected to the rays was drawn past the
ball. If by means of storage cells the ball was maintained at a
constant potential, and if the modified air was drawn continu-
ously through the tube, an electric current arose just as if

MEMOIRS ON

the ball were connected to the wall of the tube by a poor
conductor.

( /) The question arises. How does the air lose the property
which is given it by the X-rays ? It is not yet settled whether
it loses this property gradually of itself — i. e., without coming
in contact with other bodies. On the other hand, it is certain
that a brief contact with a body of large surface, which does
not need to be electrified, can make the air inactive. For in-
stance, if a thick enough stopper of wadding is pushed into the
tube so far that the modified air must pass through it before it
reaches the electrified ball, the charge on the ball remains un-
affected even while the exhaustion is taking jolace.

If the wad is in front of the aluminium window, the result
obtained is the aame as it would be without the wad; a proof
that it is not particles of dust which are the cause of the ob-
served discharge.

Wire gratings act like wadding; but the gratings must be
very fine, and many layers must be placed over each other if
the modified air is to be inactive after it is drawn through
them. If these gratings are not connected to the earth, as has
been assumed, but are connected to a source of electric-
ity at a constant potential, I have always observed exactly
what I had expected ; but these experiments are not yet com-
pleted.

{g) If the electrified bodies, instead of beiug in air, are
placed in dry hydrogen, they are also discharged by the X-rays.
The discharge in hydrogen seemed to me to proceed somewhat
more slowly; yet this is still uncertain on account of the diffi-
culty of obtaining exactly equal intensities of the X - rays in
consecutive experiments.

The method of filling the apparatus with hydrogen precludes
the possibility that the layer of air which was originally pres-
ent, condensed on the surface of the bodies, played any im-
portant r6le.

{h) In spaces which are highly exhausted the discharge of a
body by the direct incidence of X-rays proceeds much more
slowly — in one case about seventy times more slowly — than in

ROXTGEX RAYS

the same vessels when filled with air or hydrogen at atmos-
pheric pressure.

(i) Experiments are about to be begun on the behavior of a
mixture of chlorine and hydrogen under the influence of X-
rays.

( /) In conclusion I would like to mention that the results
of investigations on the discharging action of X-rays in which
the influence of the surrounding gas is not taken into account
should be received with great caution.

19. It is advantageous in many cases to include a Tesla ap-
paratus (condenser and transformer) between the discharge-
apparatus which furnishes the X-rays and the induction-coil.
This arrangement has the following advantages: first, the dis-
charge-apparatus is less easily penetrated and is less heated;
second, the vacuum maintains itself for a longer time, at least
in my self-constructed apparatus; third, many discharge-tubes
under these conditions give more intense X-rays. With tubes
which have not been exhausted sufficiently or have been ex-
hausted too much to be driven satisfactorily by the induction-
coil alone, the addition of the Tesla transformer renders good
service.

The question immediately arises — and I allow myself to men-
tion it without being able to contribute anything to its solu-
tion at present — whether X-rays can be produced by a con-
tinuous discharge under constant difference of potential; or
whether variations of this potential are essential and neces-
sary for the production of the rays.

20. In paragraph 13 of my first memoir I announced that
X-rays could originate not only in glass, but in aluminium also.
In the continuation of my ex^^eriments in this direction I have
not found any solid body which cannot, nnder the action of the
cathode rays, produce X-rays. There is also no reason known
to me why liquids and gases may not behave in the same man-
ner.

Quantitative differences in the behavior of different substances
have appeared, however. If, for instance, the cathode rays
fall upon a plate one half of which is made of platinum 0.3
B 17

MEMOIRS ON RONTGEN RAYS

millimetre thick, the other half of aluminium 1 millimetre
thick, we see on the photographic image of this double plate,
taken by means of a pin-hole camera, that the platinum sends
out many more X-rays from the side struck by the cathode
rays (the front side) than does the aluminium from the same
side. However, from the rear side the platinum emits prac-
tically no X-rays, while the aluminium sends out relatively
many. These last rays are produced in the front layers of the
aluminium and pass through the plate.

We can easily devise an explanation of this observation, yet
it may be advisable to learn other properties of the X-rays be-
fore so doing.

It must be mentioned, however, that there is a practical
importance in the facts observed. For the production of the
most intense X-rays platinum is best suited, according to my
exj)eriments up to the present. I have used for some weeks
with great success a discharge-apparatus in which the cathode
is a concave mirror of aluminium, and the anode is a plate of
platinum placed at the centre of curvature of the mirror and
inclined to the axis of the mirror at an angle of 45°.

21. The X-rays proceed in this case from the anode. I must
conclude, though, from experiments with apparatus of different
kinds that it is entirely immaterial, so far as the intensity of
the X-rays is concerned, whether the place where the rays are
produced is the anode or not.

A discharge - apparatus was prepared specially for experi-
ments with the alternating currents of the Tesla transformer ; in
it both electrodes were aluminium concave mirrors whose axes
were at right angles; at their common centre of curvature
there was placed a platinum plate to receive the cathode rays.
Further information will be given later as to the usefulness of
this apparatus.

WiJRZBDRG, Physikalisches lustitut der Universitat.
March 9, 1896.

FURTHER OBSERVATIONS ON THE PROP-
ERTIES OF THE X-RAYS

W. C. ROXTGEX

THIRD COMMUNICATION

Sitzungshericht der Koniglichen joreussischen Akademie der Wissenscliaften zu

BerUii, 1897 — Wiedemann, Annalen der Physik und

der Chemie, 64, 1898.

CONTEXTS

PAGE

Diffusion of X-Rays 21

Conditions Influencing Fluorescence 24

Intensity in Biff'erent Directions 24

Transparency ; Selective Absorption 26

Absorption with Different Tubes; with Different Interrupters, etc 30

''Hard'' and "Soft " Tubes 32

Connection between X-Bays and Cathode Bays 35

Absorption by Crystals 39

Optical Effect of X-Bays 39

Diffraction of X-Bays 40

FURTHER OBSERVATIONS ON THE PROP-
ERTIES OF THE X-RAYS

W. C. ROXTGEX

THIR D CO MM UNI CATIO N

1. If an opaqne plate be placed betvreen a discharge-appara-
tus* which is emitting intense X-rajs and a fluorescent screen,
in such a position that it shades the entire screen, there may still
be noticed, in spite of the plate, an illumination of the barium
platino-cyanide. This illumination can be seen even when the
screen lies directly on the plate; and one is inclined at first
sight to consider the plate as transparent. If, however, the
screen lying on the plate be covered by a thick pane of glass,
the fluorescent light becomes much weaker; and it vanishes
entirely if, instead of using a glass plate, the screen is sur-
rounded by a cylinder of sheet-lead 0.1 centimetre thick, which
is closed at one end by the non-transparent plate, and at the
other by the head of the observer.

The phenomenon now described may be due either to diffrac-
tion of rays of very great wave-length, or to the fact that the
bodies which surround the discharge-apparatus and tlirough
whicli the rays pass, especially the air, themselves emit X-rays.

* All the discharge-tubes mentioned in the following communicalion are
constructed according to the principle given in paragraph 20 of my sec-
ond paper. The greater portion of them I obtained from the firm of
Greiner & Friedrichs, in Stutzerbach i. Th., whom I wish to thank publicly
for the material whicli has been furnished me in such abundance and
without expense.

MEMOIRS ON

The latter explanation is the correct one, as may be proved
with the following apparatus, among others : The fig'ire rep-
resents a very thick-walled glass bell-jar, 20 centimetres high
and 10 centimetres broad, which is closed by a thick zinc plate
cemented on. At 1 and 2 are inserted
plates of lead in the shape of circular
segments ; these are somewhat larger
than half the cross -section of the jar.
and prevent the X-rays, which enter
through an opening in the zinc plate
covered with a celluloid film, from reach-
ing directly the space above the lead
plate, 2. On the upper side of this
sheet of lead there is fastened a small
barium platino - cyanide screen, which
nearly fills the entire cross-section of the
jar. This cannot be struck either by the
-* direct rays or by such as have suffered a
single diffuse reflection at a solid body {e.g., the glass wall).
The jar is filled with dust-free air before each experiment. If
X-rays are made to enter the jar in such a manner that they
are all received upon the lead screen 1, no fiuorescence is
observed at 2 ; the fluorescent screen first begins to light up
on the half not covered by the lead plate 2 only when by tip-
ping the bell-jar direct radiation reaches the space between 1
and 2. If the bell-jar is now connected to an aspirator-pump
worked by a stream of water, it is observed that the fluores-
cence becomes more and more weak as the exhaustion pro-
ceeds ; but when the air is readmitted the intensity again in-
creases.

Since now, as I have found, the mere contact with air whicli
has been exposed shortly before to X-rays does not produce any
sensible fluorescence of the barium platino-cyanide, we must
conclude from the experiment described that air during its ex-
posure to radiation emits X-rays in all directions.

If our eyes were as sensitive to X-rays as they are to light-
rays, a discharge-apparatus in operation would appear to us

ROXTGEX RAYS

like a light bnrning in a room moderately filled witli tobacco
smoke ; perhaps the colors of the direct rays and of those
coming from the particles of air might be different.

The question as to whether the rays emitted by a body which
is receiving radiation are of the same kind as those which are
incident, or, in other words, whether the cause of tliese rays is
diffuse reflection or a process like fluorescence, I have not yet
been able to decide. The fact that the rays coming from the
air are photographically active can be proved easily ; and this
action makes itself noticeable sometimes in a way not desired
by the observer. In order to guard against this action, as is
often necessary in long exposures, the photographic plates must
be protected by suitable lead casings.

2. In order to compare the intensity of the radiation of two
discharge - tubes, and for various other experiments, I have
used an arrangement which is based on the principle of the
Bouguer photometer, and which, for the sake of simplicity, I
shall call a photometer also. A rectangular sheet of lead 35
centimetres high, 150 centimetres long, and 0.15 centimetre
thick, supported on a board frame, is placed vertically in the
middle of a long table. At each side of this is placed a dis-
charge-tube, which can be moved along the table. At one end
of the lead strip a fluorescent screen* is so placed that each
half receives radiation perpendicularly from one tube only.
In effecting the measurements, adjustments are made until
there is equal brightness of the fluorescence on the two halves.

Some remarks on the use of this instrument may find a place
here. It should be mentioned first that the settings are often
made more difiicult by the lack of constancy of the source of
radiation, the tubes responding to every irregularity in the
interruption of the primary current, such as occur with the

* In this and other experiments the Edison fluorescent screen has proved
most useful. This consists of a box like a stereoscope which can be held
light-tight against the head of the observer, and whose card-board end is
covered with barium platino-cyanide. Edison uses tutigstate of calcium
in place of barium platiuo - cyanide ; but I prefer the latter for many
reasons.

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Deprez interrupter, and especially with the Foucault instru-
ment. Repeated settings are therefore advisahle. In the sec-
ond place, I should here enumerate the conditions which in-
fluence the brightness of a given fluorescent screen struck b}^
X-rays in such rapid succession that the eye of the observer
can no lonsrer detect the intermittence of the radiation. This
briglitness depends (1) wpon the intensity of the radiation
which proceeds from the platinum plate of the discharge-tube ;
{'i) very j^robably upon the kind of rays striking the screen,
since all kinds of rays (see below) are not necessarily equally
active in producing fluorescence ; (3) upon the distance of the
screen from the centre of emission of the rays ; (4) upon the
absorption which the rays experience on their way to the barium
platino-cyanide screen ; (5) upon the number of discharges
per second ; (6) upon the duration of each single discharge ;
(T) upon the duration and the strength of the after-illumina-
tion of the barium platino-cyanide; and (8) upon the radiation
falling on the screen from the bodies which surround the dis-
charge-tube. In order to avoid errors, it must always be re-
membered that the conditions are in general like those which
would exist if we had to compare, by means of fluorescent
action, two intermittent sources of light of different colors,
which are surrounded by an absorbing envelo2)e placed in a
turbid — or fluorescing — medium.

3. According to paragraph i'i of my flrst communication,
the point in the discharge -apparatus which is struck by the
cathode rays is the centre of emission of the X-rays, and from
this these rays spread out 'Mn all directions.'' It becomes
now of interest to determine how the intensity of the radiation
varies with tlie direction.

For this investigation the discharge-tubes best suited to the
purpose are those in the shape of a sphere, with smoothly
polished platinum plates, which are struck by the cathode rays
at an angle of -45°. Even without further appliances we can
recognize from the uniformly bright fluorescence of the hemi-
spherical glass wall surrounding the platinum plate that very
iiroat differences of intensity in different directions do not

RONTGEN RAYS

exist ; so that Lambert's law of emission does not hold in
this case. Xevertheless, this fluorescence for the most part
might still be due to the cathode rays.

To test this question more accurately, several tubes were ex-
amined by means of the ^^hotometer as to their radiation in
different directions. Moreover, besides doing this, I have ex-
posed with the same object photographic films bent into a semi-
circle (radius 25 centimetres) about the platinum plate of the
discharge-tube as a centre. In both experiments, however, the
varying thickness of the different portions of the walls of the
tube produced a disturbing action, because the X-rays, pro-
ceeding in different directions, were unequally absorbed. Yet
by interposing thin plates of glass I finally succeeded in mak-
ing the thickness of glass traversed about the same.

The result of these experiments is that the radiation through
an imaginary hemisphere, described around the platinum plate
as a centre, is nearly uniform almost out to the edge. It was
not until the emission angle of the rays was about 80° that I
noticed the beginning of a decrease in the radiation ; and even
then this decrease was relatively very small ; so that the main
change in the intensity occurs between 89° and 90°.

Xo difference in the kind of rays emitted at different angles
have I been able to detect.

As a consequence of the distribution of intensity of the X-
rays, as now described, the images of the platinum plate which
are received — either on a fluorescent screen or on a photo-
graphic plate, through a pin-hole camera or witli a narrow slit
— must be more intense the greater the angle which the plati-
num plate makes with the screen or with the photographic
plate ; always presupposing that this angle does not exceed
80°. By means of suitable appliances which allow comparisons
to be made between the images received simultaneously at dif-
ferent angles from the same discharge-tube, I have been able
to confirm this conclusion.

A similar case of distribution of the intensity of emitted rays
occurs in Optics in the case of fluorescence. If a few drops of
fluorescein solution be allowed to fall into a rectans^ular tank

MEMOIRS OX

filled with water, and if at the same time we illuminate the
tank with white or with violet light, we observe that the
brightest fluorescence proceeds from the edges of the threads
of the slowly sinking fluorescein — i. e., from the j^laces where
the emission angle of the fluorescent light is the greatest. As
Stokes has remarked, d propos of a similar experiment, this
phenomenon is due to the fact that the rays whicli produce
fluorescence are absorbed by the fluorescein solution much
more strongly than is the fluorescent light itself. Now it is
worthy of note that the cathode rays, which produce the
X-rays, are absorbed by platinum much more than are the
X-rays, and it is easy to conjecture from this that a relation-
ship exists between the two phenomena — the transformation of
ordinary light into fluorescent light, and that of cathode rays
into X-rays. A conclusive proof, of any kind, of such an as-
sumption is not known at the present time, however.

Moreover, with reference to the technique of the production
of shadow pictures by means of X-rays, the observations on the
distribution of intensity of the rays proceeding outward from
the platinum plate have a certain importance. According to
what has been stated above, it is advisable to place the dis-
charge-tube in such a position that the rays used in producing
the image shall leave the platinum plate at as great an angle as
possible, though this should not be much over 80°. By this
means the sharpest pictures are produced ; and, if the platinum
plate be perfectly plane, and the construction of the tube of
such a kind that the oblique rays pass through a not materially
thicker glass wall than those rays which are emitted perpen-
dicular to the platinum plate, then the radiation on the object
suffers no loss in intensity.

4. I have designated in my first communication by ''trans-
parency of a body " the ratio of the brightness of a fluorescent
screen placed perpendicular to the rays, and close behind the
body, to that which the screen shows when viewed under the
same conditions, but with the body removed. '' Specific trans-
parency " of a body will be used to indicate the transparency of
the body reduced to a thickness of unity ; this is equal to the

RONTGEX RAYS

dth root of the transparency, if d is the thickness of the layer
traversed, measured in the direction of the rays.

In order to determine the transparency, I have used prin-
cipally, since my first communication, the photometer described
above. The body to be investigated — aluminium, tin-foil, glass,
etc., made in the form of a plate — was placed before one of the
two equally bright fluorescent halves of the screen ; and the
inequality in brightness thus produced was made to vanish,
either by increasing the distance of the radiating discharge-
apparatus from the uncovered half of the screen, or by bringing
the other tube nearer. In both cases the correctly measured
ratio of the squares of the distances of the platinnm plates of
the discharge-tubes from the screen, before and after the dis-
placement of the apparatus, is the desired value of the trans-
parency of the interposed body. Both methods led to the
same result. By the addition of a second plate to the first, the
transparency of the second i)late may be found in a similar
manner for rays which have already passed throngh one.

The method above described presuj^poses that the brightness
of a fluorescent screen varies inversely as the square of its dis-
tance from the source of rays, and this is true, in the first
place, only if the air neither absorbs nor emits X-rays, and if,
secondly, the brightness of the fluorescent light is proportional
to the intensity of emission of rays of the same kind. The
first condition is certainly not satisfied, and it is doubtful
whether the second is ; I convinced myself long ago by ex-
periment, as already described in paragraph 10 of my first com-
munication, that the deviations from the law of proportion-
ality are so small that they can be safely neglected in the case
before us. It should be mentioned with reference to the fact
that X-rays also proceed from the irradiated body, first, that a
difference in the transparency of a plate of aluminium 0.9'^5
millimetre thick, and of 31 aluminium sheets laid upon one
another, each of a thickness of 0.0299 millimetre — 31 x
0.0299 = 0.927 — could not be detected with the photometer
used ; and, second, that the brightness of the fluorescent
screen was not sensibly different when the plate was close in

MEMOIRS ON

front of the screen and when it was placed at a greater dis-
tance from it.

For aluminium, the results of this experiment on trans-
parency are as follows :

Transparency for Perpendicular Rays

Tube 2 Tube 3 Tube i Tube 2

The first 1 mm. thick Al. plate 0.40 0.45 — 0.68

Thesecondlmm. '^ '^ " 0.55 0.68 — 0.73

The first 2 mm. ^' '' '' — 0.30 0.39 0.50

The second 2 mm. " *' '' — 0.39 0.54 0.63

From these experiments, and from similar ones on glass and
tin-foil, we deduce at once the following result : if we imagine
a substance divided into layers of equal thickness, placed per-
pendicular to parallel rays, each of these layers is more trans-
parent for the transmitted rays than the one before it ; or, in
other words, the specific transparency of a substance increases
with its thickness.

This result is completely in accord with what may be ob-
served in the photograph of a tin-foil scale as descril3ed in par-
agraph 4 of my first communication ; and also with the fact
that in photographic pictures the shadow of thin sheets — ('.g.,oi
the paper used to wrap up the plate — is proportionally strongly
marked.

5. Even if two plates of different substances are ecjually
transparent, this equality may not persist when the thickness
of the plates is changed in the same ratio, nothing else being
altered. This fact may be proved most easily by the help of
two scales placed side by side ; for instance, one of platinum,
the other of aluminium. I used for this purpose platinum-foil
0.0026 millimetre thick, and aluminium - foil 0.0299 milli-
metre thick. I brought the double scale before the fiuores-
cent screen, or before a photographic plate, and allowed rays
to fall upon it ; I found in one case that a single sheet of plat-
inum was of equal transparency with a six-fold layer of alumin-
ium ; but that the transparency of a double platinum layer was

RONTGEX RAYS

equal not to that of a twelve-fold layer of alumiuiimi, but to a
sixteeen-fold layer. Using another discharge-tube, I obtained,
1 platinum — 8 aluminium ; 8 platinum = 90 aluminium. It
follows from these experiments, therefore, that the ratio of the
thickness of platinum and aluminium of equal transparency is
smaller in j^roportion as the layers in question become thicker.

6. The ratio of the thicknesses of two equally transparent
plates of different materials depends also upon the thickness
and the material of the body — e.g., the glass wall of the dis-
charge-ajiparatus — which the rays must first traverse before
they reach the plates in question.

In order to prove this conclusion — which is not surprising
after what has been said in sections 4 and 5 — we may use an
arrangement which I call a platinum-aluminium window, and
which, as we shall see, may also be used for other purposes.
This consists of a rectangular piece (4.0 x 6.5 centimetres) of
platinum-foil of 0.0026 millimetre thickness, which is cement-
ed to a thin paper screen, and through which are punched 15
round holes, arranged in three rows, each hole having a diame-
ter of 0.7 centimetre. These little windows are covered with
panes of aluminium, 0.0299 millimetre thick, which fit exact-
1}-, and are carefully superposed in such a way that at the first
window there is one disk ; at the second, two, etc. ; finally, at
the fifteenth, fifteen disks. If this arrangement be brought in
front of the fluorescent screen, it may be observed very plainly,
in case the tubes are not too hard (see below), how many alu-
minium sheets have the same transparency as the platinum-
foil. This number will be called the window-number.

For the window-number I obtained in one case by direct ra-
diation the value 5. A plate of common soda -glass, 2 milli-
metres thick, was then held in front ; the window-number was
10. So that the ratio of the thickness of the platinum and alu-
minium sheets of equal transparency was reduced one -half
when I used rays which had passed through a plate of glass 2
millimetres thick instead of using those coming direct from the
discharge-apparatus. Q. E. D.

The following experiment also deserves mention in this

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place: The platinum -aluminium window was laid upon a
small package which contained 12 photographic films, and was
then exposed ; after development, the first film lying under
the window showed the window-number 10, the twelfth the
number 13 ; and the others, in proper order, the transition
from 10 to 13.

7. The experiments communicated in sections 4, 5, and G
refer to the modifications whicli the X-rays coming from a dis-
charge-tube experience on passing through different substances.
It will now be proved that one and the same substance, with
the same thickness traversed, may be transparent in different
degrees to rays which are emitted by different tubes.

In the following table are given, for this purpose, the values
of the transparency of an aluminium plate 2 millimetres thick
for rays produced in different tubes. Some of these values are
taken from the first table on page 28 :

Tbansparency for Perpendicular Radiation

Tubes

1 2 3 4 2 5

of an Al. plate 2 mm. thick, 0.0044 0.22 0.30 0.39 0.50 0.50

The discharge - tubes are not materially different in their
construction or in the thickness of their glass walls, but vary
mainly in tlie degree of exhaustion of the contained gas and in
the discharge -potential which is conditioned by this; tube 1
requires the lowest, tube 5 the highest, potential ; or, as we
shall say, to be brief, tube 1 is the *' softest,'" tube 5 the
*^ hardest." The same induction-coil — in direct connection
with the tubes — the same interrupter, and the same strength of
current in the primary were used in all the cases.

All the many other bodies which I have investigated behave
in the same manner as aluminium ; all are more transparent
for the rays of a harder tube than for those of a softer one.'*'
This fact seems to me to be worthy of special consideration.

* See below for the behavior of " nou-normal " tubes.
30

RONTGEN RAYS

The ratio of the thicknesses of two equally transparent
plates of different substances is also dependent upon the hard-
ness of the tube used. This may be recognized immediately
with the platinum-aluminium window (§ 5) ; with a very soft
tube, for example, the window-number may be found to be 2 ;
while with a tube which is very hard, but otherwise the same,
the scale which reaches No. 15 does not extend far enough.
This means, then, that the ratio of the thicknesses of platinum
and aluminium of equal transparency is smaller in proportion
as the tubes from Avhich the rays come are harder, or — with
reference to the result reported above — as the rays are less
easily absorbed.

The different behavior of rays produced in tubes of different
hardness is self-evident also in the familiar shadow-pictures of
hands, etc. With a very soft tube, dark pictures are obtained
in which the bones are not very prominent ; by using a harder
tube the bones are very plain and all the details are visible, the
soft parts, on the contrary, being weak; while with an ex-
tremely hard tube only faint shadows are obtained, even of the
bones. From what has been said it follows that the choice of
the tube to be used must depend upon the constitution of the
object to be pictured.

8. It still remains to note that the quality of the rays fur-
nished by one and the same tube depends upon a variety of
conditions. As the investigation made with the platinum-
aluminium window shows, this is influenced : (1) By the man-
ner and perfection with which the Deprez or Foucault inter-
rupter* works — i. e., by the variation of the primary current ;
to this belongs the phenomenon so often observed, that single
discharges out of a rapid succession produce X-rays which are
not only particularly intense, but which are distinguished from
the others by the [^slight^ extent to which they are absorbed ; (2)
by a spark-gap which is included in the secondary circuit of
the discharge-apparatus ; (3) by including in the circuit a Tesla

* A good Deprez interrapter works more regularly than a Foucault
apparatus ; the latter, however, utilizes the primary current better.

MEMOIRS ON

transformer ; (4) by the degree of exhaustion of the discharge-
ajjparatus (as already mentioned) ; (5) by different conditions
in the interior of the discharge-tube, which are not yet suffi-
ciently understood. Several of these factors deserve a some-
what more extended consideration.

If we take a tube which has not yet been used, nor even ex-
hausted, and connect it to the mercury-pump, we shall obtain,
after the necessary pumping and heating, such a degree of ex-
haustion that the first X-rays are noticeable by means of the
feeble illumination of the fluorescent screen lying near. A
si^ark-gap in parallel with the tube gives sparks only a few
millimetres long, the platinum-aluminium window shows only
very low numbers, the rays are easily absorbed. The tube is
*^ very soft." If now the spark-gap be put in series, or a Tesla
transformer be inserted,* rays are emitted which are more
intense and less easily absorbed. I found, for example, in one
case, that by increasing the series spark-gap the window-num-
ber could be gradually brought from 2.5 to 10.

(These observations suggested the question whether at still
higher pressures X-rays could not be obtained by the use of a
Tesla transformer. This is, in fact, the case : using a narrow
tube with wire-shaped electrodes, I could still observe X-rays
when the pressure of the enclosed air amounted to 3.1 milli-
metres of mercury. If hydrogen were used instead of air, the
pressure could be even higher. The lowest pressure at which
X-rays can be produced in air I have not been able to deter-
mine; it is in many cases less than 0.0002 millimetre of mer-
cury ; so that the limits of pressure within which X-rays may
arise are even now very considerable.)

Further exhaustion of a ^'very sof t " tube — connected di-
rectly to the induction-coil — results in the radiation becoming
more intense, and in a greater fraction of it passing through

* The fact tbat a spark-gap in series has the same effect as a Tesla trans-
former I was able to mention in the French edition of my second com-
munication {Arch, des Sci. Physique, etc., de Oen^ve, 1896); in the German
publication this remark was omitted by accident.

RONTGEN RAYS

the bodies on which it falls : a hand held in front of the fluo-
rescent screen is more transparent than before, and the plati-
nnm-aluminium window gives a higher window-number. At
the same time the spark-gap in parallel with the tube must be
increased "in length in order to send the discharge through the
tube : the tube has become " harder/' If the tube is exhaust-
ed still more, it becomes so '' hard " that the spark-gap must
be made more than 20 centimetres long ; and now the tube
emits rays for which substances are unusually transparent :
plates of iron 4 centimetres thick, for example, being seen to
be transparent when viewed with the fluorescent screen.

The behavior, as now given, of a tube directly connected
both with the pump and the induction-coil is the normal one ;
but there often occur variations which are caused by the dis-
charges themselves. The conduct of the tubes is in many
cases quite unaccountable.

We have supposed the tube to become hard owing to con-
tinued exhaustion by the pump ; this may happen in another
way. A fairly hard tube, sealed off from the pump, becomes
of itself continually harder — unfortunately for the duration of
its usefulness — even when it is used in the proper way for the
production of X-rays ; that is to say, when discharges are sent
through it which do not cause the platinum to glow, or at least
only faintly. A gradual self-exhaustion takes place.

With such a tube, which has become hard in this way, I have
obtained a most beautiful photographic shadow-picture of the
double barrels of a hunting-rifle w^th cartridges in place, in
which all the details of the cartridges, the internal faults of the
damask barrels, etc., could be seen most distinctly and sharply.
The distance from the platinum plate of the discharge -tube
to the photographic plate was 15 centimetres, the time of
exposure was 12 minutes — comparatively long owing to the
small photographic action of these rays, which are less absorb-
able (see below). The Deprez interrupter must be rej^laced by
the Foucault apparatus. It would be of interest to construct
tubes which require still higher potentials to be used than has
been possible up to the present time,
c 33

MEMOIRS ON

As to the cause of ii tube's becoming hard when sealed off
from, the pump, the explanation given above is the self - ex-
haustion of the tube owing to the discharges. But this is not
the only cause ; there are also changes at the electrodes which
influence the result. What they consist in 1 do not know.

A tube which has become too hard can be made softer by ad-
mission of air, often also by heating the tube or by reversing
the direction of the current ; or, finally, by sending powerful
discharges through it. In the last case, however, the tube has
acquired, for the most part, other properties than those men-
tioned above ; thus it often requires, for instance, a very great
discharge-potential, and yet furnishes rays which have a com-
paratively small window-number and which are easily absorbed.
I need not continue further the discussion of the behavior of
the "non-normal" tubes. The tubes constructed by Herr
Zehnder, having a vacuum which can be regulated, since they
contain a small piece of charcoal, have done me very good
service.

The observations communicated in this section, and others
also, have led me to the opinion that the composition of the
rays emitted from a discharge-tube provided with a platinum
anode is conditioned essentially upon the duration of the dis-
charge-current. The degree of exhaustion, the hardness, play
a part only because of this, since the form of the discharge de-
pends upon it. If we can produce in any way whatever the
form of discharge necessary for the appearance of the X-rays,
X-rays can be produced, and this even at relatively high
pressures.

In conclusion, it is worth mentioning that the quality of the
rays produced by a tube is not changed, or, at most, only very
slightly, by very considerable changes in the strength of the
primary current, it being presupposed that the interrupter
works the same in all cases. The intensity of the X-rays,
on the contrary, is proportional within certain limits to the
strength of the primary current, as the following experiment
shows : The distances from the discharge-apparatus at which,
in a certain case, the fluorescence of the barium platino-cyanide

RONTGEN RAYS

screen was just noticeable amounted to 18.1 millimetres, 25.7
millimetres, and 37. 5 millimetres, when the strength of the
primary current was increased from 8 to 16 to 32 amperes.
The squares of the distances are in nearly the same ratio as the
corresponding current-strengths.

9. The results stated in the last five paragraphs were derived
immediately from the individual experiments mentioned. If
we review the whole of these individual results, we reach the
following conclusions, being led to them in part by the analogy
which exists between the behavior of optical rays and X-rays :

(a) The rays emitted by a discharge - apparatus consist of a
mixture of rays which are absorbed in different degrees and
which have different intensities.

(b) The composition of this mixture of rays depends essen-
tially upon the duration of the discharge-current.

{c) The rays selected for absorption by various substances
are different for the different bodies.

(d) Since the X-rays are generated by the cathode rays, and
since both have properties in common — production of fluores-
cence, photographic and electrical action, and absorbability,
the amount of which is essentially conditioned upon the density
of the medium through which the radiation passes, etc. — the
hypothesis at once suggests itself that both phenomena are of
the same nature. AVithout wishing to bind myself uncondi-
tionally to this view, I may remark that the results of the last
few paragraphs are calculated to resolve a difficulty which has
existed in connection with this hypothesis up to the present.
This difficulty arises, first, from the great difference between the
absorption of the cathode rays investigated by Herr Lenard
and that of the X-rays ; and, second, from the fact that the
transparency of bodies for these cathode rays depends upon a
different law of the densities of the bodies from that govern-
ing the transparency for the X-rays.

As to the first difficulty, two points should be mentioned :
(1) We have seen in § 7 that there are X-rays whose absorp-
tions are very different ; and we know from the investigations
of Hertz and Lenard that the different cathode rays also differ

MEMOIRS OX

from each other with reference to their ahsorption. Even if
we admit, therefore, that the softest tube mentioned on p. 30
furnishes X-rays whose absorption is far less than that of the
cathode rays investigated by Ilerr Lenard, yet we cannot doubt
that there are X-rays which are absorbed more, and. on the
other hand, cathode rays which are absorbed less even than
those. It therefore seems perfectly possible that by further
experiments rays will be found which form, so far as absorp-
tion is concerned, the link between the one kind of rays and
the other. (2) We found in § 4 that the specific transparency of
a body is smaller in proportion as the plate traversed is thinner.
Consequently, if in our experiments we had taken plates as
thin as those of Herr Lenard, we might have obtained values
for the absorption of the X-rays which would approximate
more closely those of Lenard.

With reference to the varying influence of the density of bod-
ies on their absorption of X-rays and of cathode rays, it should
be said that this difference is found to be smaller in proportion
as more strongly absorbable X-rays are chosen for the experi-
ment (§ 7 and § 8), and in proportion as the plates traversed
are made thinner (§ 5). Consequenth^ one must acknowledge
the possibility that this difference in the behavior of the two
kinds of rays may, by means of further experiments, be made
to vanish at the same time as the differences mentioned above.

With reference to this absorbability, the rays which come
nearest to each other are the cathode rays which are especially
present in very hard tubes and the X-rays which are emitted
from the platinum plate in very soft tubes.

10. Besides exciting fluorescence, the X-rays have, as is well
known, photographic, electric, and other actions : and it is of
interest to know how far these continue parallel with each
other as the source of radiation is altered. I have been obliged
to confine myself to comparing the two actions first named.

The platinum -aluminium window is suited for this work
also. One of these is placed upon a photographic plate which
is wrapped up, a second is brought in front of the fluorescent
screen, and both are then placed at equal distances from the

ROXTGEX RAYS

discharge-apparatus. The X-rays had exactly the same media
to traverse in order to reach the sensitive layer of the photo-
graphic plate and the barium platino - cyanide. Daring the
exposure I observed the screen and determined the window-
number ; after development, the window-number was also de-
termined upon the photographic plate ; and then both num-
bers were compared. The result of these experiments is that,
using softer tubes (window-numbers 4-T), no difference could be
observed : but when harder tubes were used it seemed to me
that the window-number on the photographic plate was a little
lower, at most one unit, than that determined by means of the
fluorescent screen. This observation, however, although re-
peatedly confirmed, is not quite free from criticism, because the
determination of the high window-numbers at the fluorescent
screen is quite uncertain.

The following result is, however, entirely certain. If we
arrange, with the photometer described in § 2, a hard and a
soft tube so as to have the same brightness at the fluorescent
screen, and if a photographic plate is substituted for the
screen, we see after development of this plate that the half of
the plate which received the rays from the hard tube is con-
siderably less blackened than the other. The radiations which
produce equal intensities of fluorescence have different photo-
graphic actions.

In considering this result we must not leave out of account
the fact that neither the fluorescent screen nor the photo-
graphic plate uses up completely the incident rays ; both
transmit many rays which can again produce fluorescent or
photographic action. The result communicated holds true,
therefore, only for the thickness of the sensitive photographic
film employed and the layer of barium platino-cyanide accom-
panying it.

How very transparent to the X-rays from tubes of average
hardness the sensitive film of the photographic plate is is
shown by an experiment with 96 ''films " laid one over another,
25 centimetres distant from the source of radiation, exposed
for 5 minutes, and protected against the radiation of the air

MEMOIRS ON

by an envelope of lead. Even on the last one a photographic
action can be recognized plainly, while the first is scarcely
over-exposed. Induced by this and similar observations, I have
inquired of several firms who furnish photographic plates
whether it would not be possible to prepare plates which were
more suited for photography with X-rays than the ordinary
ones. The samples forwarded were not, however, serviceable.

I have had many oj^portunities, as mentioned already on
p. 31, to perceive that very hard tubes, under otherwise equal
circumstances, require a longer time of exposure than those
moderately hard ; this is easily understood if we remember the
result communicated in § 9, according to which all bodies so
far examined are more transparent for rays which are emitted
by hard tubes than for those coming from soft ones. The fact
that with very soft tubes the exposure must again be long is
explained by the lack of intensity of the rays emitted by \hem.

If the intensity of the rays is increased by increasing the
primary current (see p. 31), the photographic action is in-
creased in the same degree as the intensity of the fluores-
cence ; and in this case, as also in that mentioned above where
the intensity of the radiation on the fluorescent screen is al-
tered by changing the distance of the screen from the source of
the rays, the brightness of the fluorescence is proportional, or
at least nearly so, to the intensity of the radiation. This law
cannot, however, be applied generally.

11. In conclusion, I beg the privilege of mentioning the fol-
lowing isolated points :

In a discharge-tube properly made and not too soft, the X-
rays come mainly from a spot on the platinum plate struck by
the cathode rays, which is from 1 to 2 millimetres in size. But
this is not the only starting-point: the whole jilate and a part
of the wall of the tube emit rays, although to a very small ox-
tent. Cathode rays proceed from the cathode in all directions ;
their intensity, however, is important only in the neighborhood
of the axis of the concave mirror ; and therefore the most in-
tense X-rays originate on the platinum })late at the point where
this axis meets it. If the tube is very hard and the platinum

RONTGEN RAYS

thin, a great many X-rays are emitted from the rear side of the
platinum plate, and, as is shown by a pin-hole camera, from a
point which also lies on the axis of the mirror.

In these hardest tubes, also, the maximum of intensity of the
cathode rays can be deflected from the platinum plate by means
of a magnet. Some experiments on soft tubes led me to take
up again, with better apparatus, the question of the possibility
of magnetic deflection of X-rays ; I hope to be able to com-
municate soon the results of these experiments.

The experiments mentioned in my first communication on
the transparency of plates of the same thickness which are cut
from a crystal according to different directions have been con-
tinued. I have investigated plates of calcite, quartz, tourma-
line, beryl, aragonite, apatite, and barite. ^o influence of di-
rection on the transparency could be detected even with the
improved apparatus.

The fact observed by Herr Gr. Brandes, that the X-rays can
produce a light-sensation in the retina of the eye, I have found
confirmed. There stands also in my observation-] ournal a note
at the beginning of the month of November, 1895, according to
which I perceived a feeble light-sensation, which spread over
the whole field of vision, when I was in an entirely darkened
room near a wooden door on the other side of which there
was a Hittorf tube, whenever discharges were sent through the
tube. Since I observed this phenomenon only once, I thought
it a subjective one, and the fact that I never saw it repeated is
because, later, instead of the Hittorf tube, other apparatus was
used, not exhausted so much, and not provided with platinum
anodes. On account of their state of high exhaustion, Hittorf
tubes furnish rays which are only slightly absorbed, and on ac-
count of the presence of a platinum anode, which is struck by
the cathode rays, they furnish intense rays, a condition which is
favorable for the production of the light-phenomenon referred
to. I was obliged to replace the Hittorf tubes by others, be-
cause after a very short while all were perforated.

With the hard tubes now in general use the experiment of
Brandes may be easily repeated. The following description of

MEMOIRS ON RONTGEN RAYS

the mode of experimenting may be of some interest : If a ver-
tical metal slit some tenths of a millimetre broad is held as
close as possible before the open or closed eye^ and if the head,
completely enveloped in a black cloth, is then brought near the
discharge apparatus, there is observed, after some practice, a
weak, non-uniformly bright strip of light which, according to
the place where the slit is in front of the eye, takes a different
form — straight, curved, or circular. By a slow motion of the
slit in a horizontal direction, these different forms can be made
to pass gradually from one into the other. An explanation of
the phenomenon is found immediately if we consider that the
ball of the eye is cut by a lamellar sheaf of X-rays, and if we
assume that the X-rays can excite fluorescence in the retina.

Since the beginning of my work on X-rays I have tried re-
peatedly to obtain diffraction phenomena with them ; several
times I have obtained with narrow slits, etc., phenomena whose
appearance reminded one, it is true, of diffraction images ; but
when by alteration of the conditions of experiment tests were
made of the correctness of the explanation of these images by
diffraction, it was refuted in every case ; and often I could
prove directly that the phenomena had arisen in a way quite
different from diffraction. I have no experiment to describe
from which, with sufficient certainty, I could obtain proof of the
existence of diffraction of the X-rays.

WiJRZBURG, Phjsikalisclies Institut der Universitiit.
March 10, 1897.

WiLiiELM CoxRAD RoxTGEN was born March 27, 1845, in
Lennep, Rhine Province, Germany, and is at the present time
Professor of Physics at the University of Wiirzburg. He re-
ceived his doctor's degree at Zurich in 1808, and became then
an assistant to Kundt at Wiirzburg. He was finally appointed
Professor of Physics at Giessen, from which university he was
transferred to Wiirzburg. He has been engaged in many im-
portant researches which, in the main, have a bearing upon
the connection between electricity and ordinary matter.

ON THE NATURE OF THE EONTGEX RAYS

(THE WILDE LECTUEE)

Sir G. G. STOKES, Bart., M.A., LL.D., F.R.S.

(Memoirs and Proceedings of the JIanchester Literary and Philosophical
Society, 41, Part 1\., 1896-7.)

CONTEXTS

PAGE

Resume of Work by Rontgen and J. J. Thomson 43

Discussion of Properties of X-Rays 46

Discussion of Properties of Becquerel Rays 48

Discussion of Properties of Cathode Rays 49

Propagation of Pulses 54

Theory of Rontgen Rays 55

Theory of Ordinary Refraction 58

Theory of Becquerel Rays 62

Diffraction of Pulses 63

ON THE NATURE OF THE RONTGEN RAYS

(the WILDE LECTURE)
BY

Sir G. G. STOKES, Bart., M.A., LL.D., F.R.S.

Delivered July 25, 1S97.

Ever since the remarkable discovery of Professor Rontgeu
was published, the subject has attracted a great deal of atten-
tion in all civilized countries, and numbers of physicists have
worked experimentally, endeavoring to make out the laws of
these rays, to determine their nature, if possible, and to ar-
range for their application. I am sorry to say that I have not
myself worked experimentally at the subject ; and that being
the case, there is a certain amount of presumption, perhaps, in
my venturing to lecture on it. Still, I have followed pretty
well what has been done by others, and the subject borders
very closely on one to which I have paid considerable attention ;
that is, the subject of light.

In Rontgen's original paper he stated that it was shown ex-
perimentally that the seat of these remarkable rays was the
place where the so-called cathodic rays fall on the opposite wall
of the highly exhausted tube in which they are produced. I
will not stop to describe what is meant by cathodic rays. It
would take me too much away from my subject, and I may
assume, I think, that the audience I am now addressing know
what is meant by that term. This statement of Rontgen's was
not, I think, universally accepted. Some experimentalists set
themselves to investigate the point by observing the positions
of the shadows cast by bodies subjected to the discharge of the
Rontgen rays — to investigate, I say, the place within the tube
from which the rays appeared to come. Now, when the shad-

MEMOIRS Ox\

ows were received on a photographic pldte, and the shadow was
joined to the snbstance casting the shadow, and the joining
lines were produced backwards, as a rule they tended more or
less nearly to meet somewhere witliin tlie tube — Crookes' tube,
I will now call it — and some people seem to have had the idea
that at that point of meeting or approximate meeting there
was something going on which was the source of these rays. I
have in my hands a paper published in St. Petersburg by Prince
B. Galitzin and A. v. Karnojitzky, which contains some very
elaborate photographs obtained in this way. A board was taken
and ruled with cross lines at equal intervals, and at the points
of intersection nails were struck in in an upright position.
The board was placed on top of the photographic plate, with
an opaque substance between — a substance which these strange
Rontgen rays are capable of passing through, though it is
impervious to light. The shadows cast by the nails w^re ob-
tained on the photograph, and this paper contains a number
of the photographs. It is remarkable, considering the some-
what large space in the tube over which the discharge from
the cathode is spread, that the shadows are as sharp as they
actually are ; and the same thing may be affirmed of the or-
dinary shadows of the bones of the hand, for instance, which
one so frequently sees now. Another remarkable point in
these photographs is that in some cases it appears as if
there were two shadows of the same nail, as though there
were two different sources from which these strange rays
come, both situated within the Crookes' tube. Now, have
we a right to suppose that the place of meeting of the lines
by which the shadows are formed, prolonged backwards into
the tube, is the place which is the seat of action of these
rays ? I think we have not. If a portion of the Crookes'
tube which is influenced by the cathode discharge be isolat-
ed by, we will say, a lead screen containing a small hole, you
get a portion of the cathodic rays which come out through
that small hole, and you can trace what becomes of them be-
yond. It is found that the influence is decidedly stronger
in a normal direction than in oblique directions. Professor

RONTGEN RAYS

J. J. Thomson, of Cambridge, who has worked a great deal
experimentally at this subject, mentioned that to me as a
striking thing. You might imagine that the fact that the
shadows appear to be cast approximately from a source within
the tube could be accounted for in this way. Supposing, as
Rontgen believed, that the seat of the rays is in the place
where the cathode discharge falls on the surface of the glass,
those which come in an oblique direction have to pass through
a greater thickness of glass than those which come in a nor-
mal direction. Xow, glass is only partially transparent to
the Rontgen rays ; therefore the oblique rays would be more
absorbed in passing through the glass than the rays which
come in a normal direction. I mentioned that to Professor
Thomson, but he said he thought the difference between the
intensity of the rays which come out obliquely and those
which come out in a normal direction was much too great
to be accounted for in that way.* I will take it as a fact,
without entering at present into any speculation as to the
reason for it, that the Rontgen rays do come out from the
glass wall more copiously in a normal direction than in an
oblique direction. Assuming this, we can rightly say that

* I have found b}^ subsequent inquiry that the experiment referred to
was not made by Professor Thomson himself, but by Mr. C. M. McClel-
land, in the Cavendish Laboratory, and that on being recently repeated
with the same tube the efTect of the X-rays was found to be by no means
so much concentrated towards the normal to the wall of the tube as in the
former experiment. It seems likely that the difference may have been due
to use of the tube in the interval, which would have made the exhaustion
higher, and caused the X-rays given out to be of higher penetrative power,
so as to render the increased thickness of glass which the rays emerging
obliquely had to pass through to be of less consequence. But the subject
is still under examination. In consequence of the result obtained in the
second experiment, the statement in the text should be less absolute ; but
it may very well have happened that in the experiments of others the con-
ditions may more nearly have agreed with those of the first experiment,
causing w^hat we may call the resultant activity of the X-rays to have had
a direction leaning towards the normal drawn from the point casting the
shadow to the wall of the tube.

MEMOIRS ON

the results obtained by Prince Galitzin and M. v. Karnojit-
zky, and similar results obtained by others, do not by any
means prove that the seat of the rays is within the tube.
Suppose, for example, that the tube were spherical, and a por-
tion of this spherical surface were reached by the cathodic
rays; if the Rontgen rays which passed outside came wholly,
we will say, in a normal direction, produce the directions
backwards and you will get the centre of the tube. But we
have no right to say from that that there is anything particular
going on in the centre of the spherical tube. The result is
perfectly compatible with Rontgen's original assertion, which I
believe to be true, as to the seat of the rays.

Everything tends to show that these Rontgen rays are some-
thing which, like rays of light, are propagated in the ether.
What, then, is the nature of this process going on in the ether ?
Some of the properties of the Rontgen rays are very surprising,
and very unlike what we are in the habit of considering with
regard to rays of light. One of the most striking things is the
facility with which they go through bodies which are utterly
opaque to light, such, for example, as black paper, board, and
so forth. If that stood alone it would, perhaps, not constitute
a very important difference between them and light. A red
glass will stop green rays and let red rays through ; and just in
the same way if the Rontgen rays were of the nature of the or-
dinary rays of light, it is possible that a substance, although
opaque to light, might be transparent to them. So, as I say,
that remarkable property, if it stood alone, would not neces-
sarily constitute any great difference of nature between them
and ordinary light. But there are other properties which are
far more difficult to reconcile with the idea that the Rontgen
rays are of the nature of light. There is the absence, or almost
complete absence, of refraction and reflection. Another re-
markable property of these rays is the extreme sharpness of the
shadows which they cast when the source of the rays is made
sufficiently narrow. The shadows are far sharper than those
produced under similar circumstances by light, because in the
case of light the shadows are enlarged as the effect of diffrac-

ROXTGEX RAYS

tiou. This absence, or almost complete absence, of diffraction
is then another circumstance distinguishing these rays from or-
dinary rays of light. In face of these remarkable differences,
those who speculated with regard to the nature of the rays were
naturally disposed to look in a direction in which there was
some distinct difference from the j^rocess which we conceive to
go on in the propagation and production of ordinary rays of
light. Those who have speculated on the dynamical theory of
double refraction have been led to imagine the possible exist-
ence in the ether of longitudinal vibrations, as well as those
transversal vibrations which we know to constitute light. If
we were to suppose that the Rontgen rays are due to longitudi-
nal vibrations, that would constitute such a very great difference
of nature between them and rays of light that a very great dif-
ference in properties might reasonably be expected. But as-
suming that the Rontgen rays are a process which goes on in
the ether, are the vibrations belonging to them normal or trans-
versal ? If we could obtain evidence of the polarization of
those rays, that would prove that the vibrations were not nor-
mal, but transversal. But if we fail to obtain evidence of polar-
ization, that does not at once prove that the vibrations may not
after all be transversal, because the properties of these rays are
such as to lead us dpiiori to expect great difficulties in the way
of putting in evidence their polarization, if, indeed, they are
capable of polarization at all. Several experimentalists have
attempted, by means of tourmalines, to obtain evidence of
polarization, but the result in general has been negative. Of
the two photographic markings that ought to be of unequal
intensity on the supposition of polarization, one could not say
with certainty that one was darker than the other. Another
way of obtaining polarized light is by reflection at the proper
angle from glass or other substance ; but, unfortunately for the
success of such a method, the Rontgen rays refuse to be regu-
larly reflected, except to a very small extent indeed. The au-
thors of the paper to which I have already referred appear to
have had some success with the tourmaline. Like others who
have worked at the same experiment, they took a tourmaline

MEMOIRS ON

cut parallel to the axis and put on top of it two othere, also cut
parallel to the axis, and of equal thickness,, which were placed
with their axes parallel and perpendicular respectively to that
of the under tourmaline. But they supplemented this method
bv a device which is not explained in the paper itself, although
a memoir is referred to in which the explanation is to be found
— at least by those who can read the Bussian language, which,
unfortunatelv, I cannot. I can, therefore, only guess what the
method was. It is something depending on the superposition of
sensitive photographic films. I suspect they had several photo-
graphic films superposed, took the photographs on these, and
then took them asunder for development, and after develop-
ment put them together again as they had been originaUy.
They consider that they have succeeded in obtaining evidence
of a certain amount of polarization. If we assume that evi-
dence to be undoubted, it decides the question at once. But
as the experiment, as made in this way, is rather a delicate
one. it is important for the evidence that we should consider as
well what we may call the Becquerel rays. If time permits, I
shall have something to say abont these towards the close of my
lecture, but, for the present, I shall say merely that they ap-
pear to be intermediate in their properties between the Bontgen
rays and rays of ordinary light. The Becquerel rays undoubt-
edly admit of polarization, and the evidence appears on the
whole pretty conclusive that the Bontgen rays, like rays of or-
dinary light, are due to transversal, and not to longitudinal,
vibrations. It remains to be explained, if we can explain it,
wherein lies the difference between the nature of the Bontgen
rays and rays of ordinary light which accounts for the strange
and remarkable difference in the properties of the two. I may
mention that, although Cauchy and Xeumann. and some others
who have written on the dynamical theory of double refraction,
have been led to the contemplation of normal vibrations. Green
has put forward what seems to me a very strong argument
against the existence of normal vibrations in the case of light.
The argument Green used always weighed strongly with me
against the supposition that the Bontgen rays were doe to

RONTGEX RAYS

longitnclinal vibrations ; and the experiments bj which, as I
conceive,, the possibility of their polarization has now been
established go completely in the same direction, showing that
they are due, assuming them to be some process going on in
the ether, to a transversal disturbance of some kind.

Xow, the so - called cathodic rays are, as we may say, the
parents of the Rontgen rays. Consequently, if we are to ex-
plain the nature of the Rontgen rays, it is very important that
we should have as clear ideas as may be permissible of the nat-
ure of the cathodic rays. Xow, two views have been enter-
tained as to the nature of the cathodic rays. According to one
view, they are not rays of light at all, but streams of molecules
which are projected from the cathode, and, if the exhaustion
within the tube be sufficient, reach the opposite wall. That
was the idea under which Crookes worked in his well-known
experiments, and, so far as I know, it is the view held by all
physicists in this country. Another opinion, however, has
been published, and there are some eminent physicists who
favor it, especially, I think, in Germany. According to this
latter opinion, the cathodic rays are, like rays of light, some
process going on in the ether. The cathodic ray, coming from
the cathode towards the opposite wall of the tube, is invisible
as such if you look across it. There is in reality a faint blue
light ordinarily, but not necessarily, seen when you look across
it. Lenard, in his most elaborate and remarkable experiments,
succeeded in producing the cathodic rays within a space from
which the gas was so very nearly completely taken away
that, although the cathodic rays passed freely through the
space, there was no appearance of the blue light when you
viewed their path transversely. They produced, however,
the ordinary effect of phosphorescence at the other end of
the tube. The appearance, then, may be analogous to that of
a sunbeam coming from a hole in the clouds. If it were not
for the slight amount of dust and suspended matter in the air,
the sunbeam would be invisible if you looked across it. But as
the air is never free from motes, you see the path of the sun-
beam vrhen you look across it by the light reflected from these
D 49

MEMOIRS OX

motes. Something of the same kind may be conceived to take
place with regard to the cathodic rays if they are some process
going on in the ether. But there are very great difficulties in
the way of this second hypothesis, and especially as regards
certain properties of the cathodic rays. In the first place, they
act mechanically. In Crookes' experiments he succeeded in
causing a light windmill, if I may so describe it, to spin rapidly
under the action of the rays. And when they were received
on a very thin film of blown glass, the glass was actually bent
under them as they fell upon it. But that is not all. These
cathodic rays appear to proceed in a normal direction from the
cathode, and ordinarily proceed in straight lines. But — and
this is the important point — they are capable of being deflected
in their path both by electro-static force and by magnetic or
electro-dynamic force. Xothing whatever of the kind occurs
with rays of light, and there are enormous, almost insuj^erable,
difficulties in the supposition of any such deflection occurring
if the cathodic rays are a process going on in the ether. I will
not go into all the arguments for and against the two views,
especially as the cathodic rays only enter incidentally into the
subject I have chosen to bring before you. I will confine my-
self to one or two of the chief difficulties in the way of the sup-
position that the cathodic rays are streams of molecules. In
his admirable experiments Lenard produced the cathodic rays
in a tube which was highly exhausted, but not exhausted to
the very highest degree that art can obtain. When you get to
such tremendous exhaustions as that you cannot get the dis-
charge to pass through the tube. "What did he do ? Previous
experiments had shown that certain metals — aluminium espe-
cially— are^ or appear to be, to a certain extent transparent to
these rays. AVorking on the supposition that an aluminium
plate is, to a certain extent, transparent to these rays, Lenard
constructed a tube, highly exhausted, but not to the very last
degree. .Then a window of aluminium-foil — a very small aper-
ture for mechanical reasons — was fastened in an air-tight man-
ner at the end of the tube, to lead into a second tube provided
with a phosphorescent screen. The cathodic rays produced in

RONTGEX RAYS

the first tube fell upon the aluminium plate and, as Lenard
su23posed, passed through it as rays of light would pass through
glass. And so he got them into the second tube, and, it not
being necessary to make an electric discharge ^^ass through the
second tube, he could exhaust it to the very highest power of
skill that he had. It was a work of days and days. The ca-
thodic rays behaved in this very highly exhausted tube like ordi-
nary cathodic rays. We are asked to assume that we are dealing
here with a vacuum, and according to Lenard that shows — and
no doubt it would if we grant the assumption — that it is no
longer a question of matter, but of some process going on in
the ether.* And, apparently on the strength of that very
elaborate experiment, Rontgen in his first paper seems to have
been of the opinion that the cathodic rays were something go-
ing on in the ether. But are we justified in assuming that we
are here dealing with a perfect vacuum ? I do not think we
are. I believe it passes the power of art to produce a perfect
vacuum. You always have a little residue of which yott cannot
absolutely get rid, and some of Lenard's own figures show the
effect of the residual gas. He isolated by screens a small part
of the cathodic discharge in the second tube, and received it
on a phosphorescent screen. He represents the phosphores-
cent light in the tube as consisting of a bright nucleus sur-
rounded by a less bright halo. The bright nucleus was such as
would be produced if the cathodic rays were rays of light, pro-
vided that that light were incapable of diffraction. But, then,
how do you account for the halo ? The blue light by which
the cathodic rays are seen under ordinary circumstances is due,
I believe, to an interference of the projected molecules with
the molecules of the gas. In some of Lenard^s experiments he
received the cathodic rays in the first tube into the air, and a
considerable amount of this blue light was seen. The appear-
ance was much as if you had admitted a beam of light into a

* Even if the vacuum were perfect, and the result were still the same,
that would uot disprove the theory that the cathodic rays are streams of
molecules, for the molecules might have been obtained from the alumin-
ium window itself.

MEMOIRS ON

mixture of milk and water. To my mind this fainter halo in
the most refined of Lenard's experiments, lying outside this
well-defined nucleus, was evidence that the vacuum, in spite
of all the skill and time expended upon it, was not perfect.
And for aught we know to the contrary — I believe, indeed, it
is the case — the cathodic rays in the second highly exhausted
tube were really streams of molecules coming from the residual
gas in the tube. But now comes a difficulty with regard to the
passage of the cathodic rays through an aluminium plate. If
the cathodic rays were something going on in the ether we might
very well understand that an aluminium plate might be trans-
parent to them although opaque to ordinary rays of light. But
if the cathodic rays are really streams of molecules, how can
we imagine that they get through the plate ? Do they get
through the plate ? I do not believe they do. Do they riddle
the plate like a bullet going through a thin piece of board ? I
do not think it. Suppose you have a trough containing a solu-
tion of sulphate of copper, and at the ends of it you have two
copper plates ; if you send an electric current through the
trough, copper is eaten away at the anode and deposited at the
cathode. Now, suppose you divide this trough into two by a
plate of copper, you still have copper eaten away at the original
anode and copper deposited at the original cathode. The in-
terposed plate really divides the cell into two, in each of which
electrolysis goes on, so that you have not only copper eaten
away at one end of the trough and deposited at the other, but
in your interposed plate you have copper eaten away at one
side and deposited at the other. So it may be that the second
surface of the. aluminium-foil becomes, as it were, a new cath-
ode, and starts cathodic rays. This, perhaps, is not what we
should have anticipated beforehand. Still, there is nothing
unnatural in it, and nothing, it seems to me, in consequenQ.e
of which you would be obliged to reject the theory which
makes the cathodic rays to be streams of molecules. There
are one or two other difficulties mentioned by Wiedemann, but
I do not think they are at all serious ; they are certainly not
so serious as the one I have just referred to. I will, therefore,

ROXTGEN RAYS

pass on. The possibility of deflecting the cathodic rays by
electrostatic and magnetic forces seems to be an insuperable
difficulty in the way of the theory which makes them to be a
process going on in the ether; but both of these are perfectly
in accordance with what was to be expected on the supposition
that they are streams of molecules, provided you remember
that these molecules are highly charged with electricity. A
moving charged body behaves as regards deflection like an
electric current. Again, if you have highly charged molecules
in the neighborhood of a positively or negatively statically
charged body, they will be attracted or repelled, and the de-
flections of the rays are precisely what was to be expected ac-
cording to that theory. I think we may assume that the
cathodic rays are really streams of electrified molecules which
strike against the opposite wall of the tube, or, as I will now
call it, the target. Now, when a molecule, coming in this
way from the cathode, strikes the target, how does the mole-
cule act ? It may act in two ways. It may act as a mass of
matter, infinitesimal though it be, by virtue of its momentum
— by virtue of its mass and velocit}^ — and it may act also as a
charged body, a statically charged body. What the appropri-
ate physical idea is of a statically charged body is more than I
can tell you. I was talking not long ago to Lord Kelvin about
it — and he is a far higher authority in electrical matters than I
am — and he considers that the physical idea of a statically
charged body is still a mystery to us. Well, if these charged
molecules strike the target we may think it exceedingly prob-
able that by virtue of their charge they produce some sort of
disturbance in the ether. This disturbance in the ether would
spread in all directions from the place of disturbance, so that
each projected molecule would on that supposition become, on
reaching the target, a source of ethereal disturbance spread-
ing in all directions. Well, what is the character of such a
disturbance ? The problem of diffraction, dynamically consid-
ered, may be supposed to reduce itself to this. Suppose you
have an infinite mass of an elastic medium, and suppose a
small portion is disturbed in the most general way possible,

MEMOIRS ON

what will take place ? A wave of disturbance will spread out
spherically from the place of disturbance.* You might at first
sight suppose that you could have a wave, in any limited region
of which you might have a transversal disturbance in some one
direction, the same all through the thickness of the shell oc-
cupied by the wave, though naturally the direction of disturb-
ance might vary from one region to another more or less dis-
tant region. But the dynamical theory shows that that is not
possible. In any limited region, or elementary area, as we may
regard it, of the wave, as you pass in a direction perpendicular
to the front, the disturbance in one direction must be ex-
changed for a disturbance in the opposite direction, in such a
manner that ultimately — that is, when the radius of the wave
is very large compared with its thickness — the integral of the
disturbance in one direction, which we may designate as posi-
tive, must be balanced by the integral of the disturbance in
the opposite, or negative, direction. The simplest sort of
'^ pulse," as I will call it, in order to distinguish it from a peri-
odic undulation, would be one consisting of two halves in
which the disturbances were in opposite directions. The pos-
itive and negative parts are not necessarily alike, as one may
make up by a greater width, measured in the direction of
propagation, for a smaller amplitude ; but it will be simplest
to think of them as alike, except as to sign. The following
figure represents this conception, the
positive and negative halves being dis-
tinguished by a difference of shading.
According to the view here put forward, the Rontgen
emanation consists of a vast succession of independent pulses,
starting respectively from the points and at the times at
which the individual charged molecules projected from the
cathode impinge on the target. At first sight it might ap-
pear as if mere pulses would be inadequate to account for

* If tlie medium be compressible there will be two waves, that which
travels the more swiftly consisting of normal vibrations ; but the opinion
has already been expressed that it is transversal vibrations with which we
are concerned.

ROXTGEN RAYS

the effects produced, seeing that in the case of light we
have to deal with series consisting each of a ver}^ great
number of consecutive undulations. But we must bear in
mind how vast, according to our theoretical views, must be
the number of molecules contained in the smallest quantity
of ponderable matter of which we can take cognizance by our
senses. Hence, small as is the quantity of matter projected
in a given short time from the cathode, it may yet be suffi-
cient to give rise to pulses the number of which is inconceiv-
ably great. It remains to consider in what way this concep-
tion may enable us to explain the most striking properties of
the Eontgen rays in relation to the contrasts which they offer
to rays of light.

The most elementary difference, as being one which has re-
lation only to propagation in the ether, consists in the absence,
or, at any rate, almost complete absence, of diffraction. As
the different pulses are by hypothesis quite independent of
one another, we have to exj^lain this phenomenon for a single
pulse.

In the figure let CB be a portion of a spherical pulse spread-
ing outwards from the centre of disturbance (which I will call
0) from which it came, P a point in
front of the wave, where the disturb-
ance which will arrive there is sought.
From P let fall a normal PQ on the
front of the wave, and let AB, taken
around Q, be a small portion of the
spherical shell which at the present
moment is the seat of the pulse, and suppose the breadth of
AB to be small compared with PQ and with the radius of
the shell, but large compared with the shelFs thickness. Let
CD be an element of the shell of similar size to AB, but sit-
uated in a direction from P distinctly inclined to PQ ; and
supposing all the disturbance in the shell stopped except what
occupies one or other of the elements AB, CD, let us inquire
what will be the disturbance subsequently produced at P in
the two cases respectively.

MEMOIRS OX

I have shown elsewhere* that in onr present problem the
disturbance at P is expressed by a double integral taken over
such portion of the surface of a sphere with P for centre and
U for radius {b being the velocity of propagation) as lies with-
in the disturbed region, which in this case is the spherical
shell or a part of it. It will be convenient to think of a series
of spheres drawn round P with radii M for increasing values
of t. When t is such that the sphere just touches the shell at
Q, and then goes on increasing, the disturbance is nearly the
same all over that portion of the surface of the sphere which
lies within the small region AB, and that, whether we take
the portion of the expression for the disturbance at P which
depends on the disturbance (displacement or velocity) at the
surface of the sphere whose radius is bt, or the portion which
depends on the differential coefficient of the displacement or
velocity with respect to a radius vector drawn from 0. Con-
sequently the positive and negative parts of the disturbance
will reach P in succession. But if instead of the small portion
AB of the shell we take CD, lying in a direction from P not
very near the normal, it is easy to see that the positive and
negative parts of the disturbance expressed by our double in-
tegral, reaching as they do P simultaneously, almost complete-
ly cancel each other. And this cancelling is so much more
nearly complete as the obliquity is greater, and likewise as the
thickness of the shell is smaller. If, then, the disturbance in
the ether consequent on the arrival of any projected molecule
at the target is very prompt, lasting, it may be, only a very
small fraction of the period of a single vibration of the etlier
in the case of light, our shell will be so thin that a small iso-
lated portion of the Rontgen discharge is propagated so nearly
wholly in the direction of a normal to the wave that the almost
complete absence of diffraction is thus accounted for. f

* "On the Dynamical Theory of Dlffraclion," Cambridge Philosophical
Transactions, vol. ix., p. 1 ; or Collected Papers, vol. ii., p. 243, Arts, 19-22.

f It is known that there is a difference of quality in Rontgen rays, and
that the Rontgen discharge may be filtered by absorption. It is known
also that the increased exhaustion in a Crookes' tube, which is accompanied

ROXTGEX RAYS

The explanation which has just been given of the apparent
absence of diffraction in the case of the Rontgen rays is closel}'
analogous to the ordinary explanation of the existence of rays
and shadows. It differs, however, in this respect, that here
we are dealing with a single pulse, whereas in the case of light
we are dealing with an indefinite succession of disturbances.
In order to understand the sharpness of the shadows produced
by the Rontgen rays, we are not obliged to suppose that the
disturbance is periodic at all. It must be partly negative and
partly positive, and that being the case, if the thickness of the
shell is very small, the amount of diffraction will be very small,
too. Those who have attempted to obtain evidence of the dif-
fraction of the Rontgen rays have been led to the conclusion
that if the rays are periodic at all the period is something enor-
mously small — perhaps thirty times, perhaps a hundred times,
as small as the wave-length of green light. It seems difficult
to imagine by what process you could get such very small vi-
brations, if vibrations there be. It is easier to understand how
the arrival of charged molecules at the cathode might produce
disturbances which are almost abrupt.

Well, then, this is what I conceive to constitute the Rontgen
rays. You have a rain of molecules coming from the electri-
cally charged cathode, which you may think of as the rain-drops
in a shower. They strike successively on the target, each mole-
cule on striking the target producing a pulse, as I have called
it, in the ether, which is essentially partly positive and partly
negative ; and you have a vast succession of these pulses coming
from the various points of the target which are not protected
by some screen interposed for the purpose of experiment.

by increasing difficulty in sending a discbarge through it, has the effect
of giving rise to increasing penetrative power in the Rontgen rays which it
gives out. It seems to me probable that this difference of qualify corre-
sponds to a more or less close approach to perfect abruptness in the pro-
duction of disturbance in the ether when a molecule propelled from the
cathode reaches the target, and accordingly to a less or a greater thickness
in the outward-travelling shell of disturbance in the ether; and that at rela-
tiveh' high exhaustions the molecules are propelled with a higher velocity,
and so give rise to a more prompt disturbance when they reach the target.

MEMOIRS ON

Tliis explains tlie absence, or almost complete absence, of
diffraction. But that is not all we have to explain ; we have
still a very serious thing behind. What is it that constitutes
the difference between the Rontgen rays and rays of ordinary
light in consequence of which the one are not refracted, or
only in an infinitesimal degree, while the other are freely re-
fracted ? This difficulty led me to conceive of a theory, which
I believe to be new, as to the nature of refraction itself — as to
the nature of what takes place, for example, when light is re-
fracted through a prism. Suppose we have light of a definite
refrangibility, and a prism on which it may be made to fall.
When the light is admitted we commonly imagine — at least, I
believe so — that the light is immediately refracted, and with
proper appliances you get the spectrum. Immediately ? I do
not think so. How is it that light travels more slowly through
refracting medium than through vacuum ? There are different
conjectures which have been advanced. One is that the ether
within refracting media is more dense than the ether in free
space. Another is that while the density is the same the elas-
ticity is less. Then, there have been speculations as to the
ether being loaded with particles of matter.

Take a piano. If you strike a note a string is set in vibration.
You would hardly hear any sound at all if it were rigidly sup-
ported. But it rests on a bridge communicating with a sound-
ing-board, and the sounding-board presents a broad surface to
the air, and is set in motion by the string. The sounding-
board and the string form a compound vibrating system. In
the same way it may be that the molecules of the glass, or other
refracting medium, and the ether form between them a com-
pound vibrating system, and, ivhen the motion is fully estab-
lished, the two vibrate harmoniously together. But how does
it get to be established ? We can hardly imagine otherwise
than that the ether is excessively rare compared with ponder-
able matter.* Well, supposing the ethereal vibrations start and

* The views as to the nature of refraction, wliicii I have endeavored to
explain, lead me incidentally to make a remark on another subject not, in-
deed, very closely connected with it. From the first, ROntgen recognized as

ROXTGEX RAYS

reach a set of molecules, they are somewhat impeded by the
molecules, and they tend also to move the molecules. But as
the molecules are relatively very heavy, it may he that it takes

the seat of the X-rays which he had discovered the place where the cathodic
rays fall on the wall of the Crookes' tube. This place is indicated to the
eye by the fluorescence of the glass. But we are not on that account to
regard the fluorescence as the cause of the Rontgen rays, or even to regard
the Rontgen emission as a sort of fluorescence. I have seen it remarked,
as indicating no very close connection between the two, that with a me-
tallic target we have a copious emission of Rontgen raj-s though there is no
fluorescence, and that when a spot on the ghiss wall of a Crookes' tube has
for some time been exposed to a rather concentrated cathodic discharge,
though the fluorescence which it exhibits under the action of the cathodic
discharge becomes comparatively dull, as if the glass were in some way
fatigued for fluorescence, it emits the Rontgen rays as well as before.

Fluorescence is undoubtedly indicative of a molecular disturbance ; but
in what precise way this disturbance is brought about by the cathodic dis-
charge is a matter on which I refrain from speculating. But whatever be
the precise nature of the process, it seems pretty evident that it can only
be by repeated impacts of molecules from the cathode that a sufficient
molecular disturbance can be got up to show itself as a visible fluores-
cence.

Suppose a shower of molecules from the cathode to be allowed suddenly
to fall on the anti-cathode, and after raining on it for a little to be as sud-
denly cut off. According to the views I entertain as to the nature of the
Rontgen rays, the moment the shower is let on the emission of Rontgen
rays begins, it lasts as long as the shower, and ceases the moment the show-
er is cut off. But the fluorescence onl}- graduall3% quickly though it may
be, comes on when the shower is allowed to fall, and gradually fades awaj--
when the shower is cut off. So far from the fluorescence being in any way
the cause of the Rontgen emission, there seems reason to think that if it
exercises any effect upon it at all, it is rather adverse than favorable. For
it has been found that when the target is metallic, and gets heated, the
Rontgen discharge falls off ; and fluorescence, like a rise of temperature,
involves a molecular disturbance, though the kind of disturbance is differ-
ent in the two cases.

As the fluorescence of the glass wall and the emission of X-rays are
two totally different effects of the same cause — namely, the molecular
bombardment from the cathode — the intensity of the one must by no means
be taken as a measure of the intensity of the other, even with the same
tube. The former effect would appear to be the more easily produced.
Tlds consideration removes a difficulty mentioned at page 10 of the paper

MEMOIRS OX

some considerable time for the molecules to be set sensibly in
motion. Xow if the system of molecules is exceedingly com-
plex, a mode of motion of the molecules, or it may be of the
constituent parts of the molecules, may be found such that
the system tends to vibrate in practically any periodic time
that you may choose ; only as you choose one time or another
the mode of vibration will be different ; and, again, according
to the direction in which the molecules are successively made
to vibrate the actual mode of vibration will be different. Well,
I conceive that the difference between the propagation of the
Runtgen rays and rays of ordinary light with reference to pass-
ing through a prism depends upon that. When you let a ray
of light fall upon a refracting medium such as glass, motions
begin to take place in the molecules forming the medium. The
motion is at first more or less irregular ; but the vibrations
ultimately settle down into a system of such a kind that the
regular joint vibrations of the molecules and of the ether are
such as correspond to a given periodic time, namely, that of
the light before incidence on the medium. That particular
kind of vibration among the molecules is kept up, while the
others die away, so that after a prolonged time — the time occu-
pied by, we will sky, ten thousand vibrations, which is only
about the forty-thousand-millionth part of a second — the mo-
tion of the molecules of the glass has gradually got up until
you have the molecules of the glass and the ether vibrating
harmoniously together. But in the case of the Rontgen rays,
if the nature of them be what I have explained, you have a
constant succession of pulses independent of one another.
Consequently there is no chance to get up harmony between
the vibrations of the ether and the vibrations of the body.
Go back to the case of light passing through glass. When

by Prince Galitzin and M. v. Karnojitzky, as attending the supposition
that the X-rays originate in the points in which the cathodic rays fall on
tlie wall of the tube or other target. Nor need it surprise us that in some
cases the shadows seem to indicate more than one source of action, when
we remember that from a given point more tbrin one normal can be drawn
to a given closed surface.

rOntgen rays

the regular combined vibration is established you have a ki-
netic energy, due partly to the motion of the ether and partly to
the motion of the molecules. If you make abstraction of the
loss of energy by reflection, the rate at which the energy passes
within the glass must be the same as it has outside, and conse-
quently there must be the same energy for one wave length,
which corresponds to one period of the vibration, inside as out-
side. But if the kinetic energy of the ether is the same for
the same volume inside and outside, and you have in addition
Inside a certain amount of kinetic energy due to the motion of
the molecules, the two taken together can only make the en-
ergy for a wave inside the same as for a wave outside on the
condition that the velocity of propagation inside is less than
the velocity of propagation outside. That is the theory I have
been forced to adopt as to the nature of refraction in conse-
quence of the ideas I hold as to the nature of the Rontgen
rays ; and if you adopt that theory I think everything falls into
its place. When you have the Rontgen rays falling on a body,
the motion of the ether due to them is interfered with by the
molecules of the body, more or less. Xo body is perfectly
transparent to these rays, and, on the other hand, perhaps we
may say no body is perfectly opaque. That all falls into its
place on this supposition as to the nature of the action of the
ether on the molecules. Xow, why is it that the Rontgen
rays do not care whether you present them with black paper or
white paper ? What is the cause of blackness ? The light
falling upon the paper produces motion in the ultimate mole-
cules. In the case of a transparent substance you have a com-
pound vibrating system going on, vibrating without change.
But in the case of an absorbing medium the vibrations which
after a time are produced in the molecules spread out into
adjoining molecules, by virtue of the communication of the
molecules with one another, and are carried away ; so that in
the case of an absorbing medium there is a constant beginning
to set the molecules in vibration ; but they never get to the
permanent state, because the vibration is carried away b}^ com-
munication from one molecule to another. But in the case of

MEMOIRS OX

the Rontgen rays you have done with the pulse altogether
long before any harmonious vibration between the ether and
the molecules can be established ; so that a state of things is
not brought about in which you get a, comparatively speaking,
large vibration of the molecules. Consequently, the Rontgen
rays do not care whether you give them black paper or not.

I must not keep you more than a minute or two longer ; but
I do not like to close this lecture without saying a word or
two regarding the Becquerel rays. What takes place there ?
To be brief, I must refer to the most striking case of all.
Take the case of metallic uranium. That gives out something
which, like the Rontgen rays, has an influence passing through
black paper, and capable of affecting a photographic plate.
It is also capable of effecting the discharge of statically charged
electrified bodies. Apparently this goes on indefinitely. You
do not need, apparently, to expose the metal to rays of high
refrangibility in order that this strange thing should go on.
What takes place ? My conjecture is that the molecule of
uranium has a structure which may be roughly compared to a
flexible chain with a small weight at the end of it. Suppose
you have vibrations communicated to such a chain at the top ;
they travel gradually to the bottom, and near the bottom pro-
duce a disturbance w^iicli deviates more from a simple har-
monic undulation. So, if a vibration is communicated to
what I win call the tail of the molecule of uranium, it may
give rise to a disturbance in the ether which is not of a regular
periodic character. I conceive, then, that you have vibrations
produced in the ether, not of such a permanently regular char-
acter as would constitute them vibrations of light, and yet not
of so simple a character as in the Rontgen rays — something be-
tween. And accordingly there is enough irregularity to allow
the ethereal disturbance to pass through black i)aper, and
enough regularity on the other hand to make possible a cer-
tain amount of refraction. You can also obtain evidence of
the polarization, and, consequently, of the transverse character
of these rays.

According to the theory of the nature of the Runtgen rays

RONTGEX RAYS

which I have endeavored very briefly to bring before you, we
have here, as I think, a system the various parts of which fit into
one another. You start with the Rontgen rays, which con-
sist, as I conceive, of an enormous succession of independent
pulses ; you pass to the Becquerel rays, which are still irregu-
lar, but are beginning to have a certain amount of regularity ;
and you end with the rays which constitute ordinary light.
According to this theory, the absence of diffraction in the
Rontgen rays is explained, not by supposing they are rays of
light of excessively short wave length, but by supposing they
are due to an irregular repetition of isolated and independent
disturbances. So far as I know, the view I have been led to
form as to the nature of refraction, and which forms an inte-
gral portion of the theory as to the Rontgen rays, is altogether
new, so much so that I felt at first rather startled by it ; but
I found myself fairly driven to it by the ideas I entertain as to
the nature of the Rontgen rays, and I am not aware of any se-
rious objection to it.

Additional Note

The problem of diffraction in the case of a vast system of
inde])endent very slender pulses deserves to be treated in some-
what greater detail. It is rather simpler than the problem of
diffraction in the case of series of undulations such as those
which constitute light, because the pulses are to 'be treated
separately and independently, like streams of light from differ-
ent sources ; and as the whole thickness of a pulse in the case
of the, Rontgen rays may probably be something comparable
with the millionth of an inch, we have no need to inquire what
will be the disturbance continually passing across a fixed sur-
face in space ; we may treat the shell at any moment as consti-
tuting an initial disturbance in the ether, and then examine the
efficiency of different parts of the shell in disturbing at a future
time the ether at a given point of space in front of the shell.

The thickness of the shell is not necessarily the same at
points situated in widely different directions as regards their
bearing from the centre, and the same applies to the direction

MEMOIRS ON

of disturbance. But in any case for a small portion of the
shell the thickness may be deemed uniform, and the direction
of disturbance sensibly the same as we pass from point to point
in a direction tangential to the shell, while it varies with great
rapidity, at least as regards its amount, when we pass from
point to point in a normal direction, vanishing at the outer and
inner boundaries of the shell.

As the disturbance we are concerned with is of the distor-

tional kind only, the disturbance
at time t at a point P in front
of the shell may be obtained
from that at time 0 in the shell
in its position wdiich is taken as
initial by the last equation in Art.
'Z'l of my paper on diffraction al-
ready cited. Let E be a point in
the shell of disturbance when in
that position which is regarded
as initial, r, r' the distances PR,
OR ; d, 6' their inclinations to
OP ; (p the azimuth round OP
of the plane PRO. Then in the
formula referred to da = sin ddddcp. Also rddxs'm {6-^d')=dr';
and sin 61/sin {6 + e')-r'IOP = r'/(r + r') very nearly.

Let OP cut the inner boundary of the shell in S, and let
ab or QS, the thickness of the shell, be denoted by X. In the
equation referred to, the term arising from the differentiation
with respect to t of the t outside the sign of double integration
will be of the order X/r' as- compared with the others, and may,
therefore, be neglected. Tlie t outside may be replaced by
r/b, and the fraction r/(r-\-r'), being sensibly constant over
the range of integration, may be put outside. Our expression
then becomes

-^^=Fi7//("-^S):r

/.p.

* Tlie suffix bt means that tliu intcgraliou is taken over a spherical sur-
face with centre Pand radius bt.

RONTGEN RAYS

As the disturbance deemed initial was only a momentary con-
dition of a wave that had been travelling outwards with the

velocity h, we must have iiQ=--h-—^^, and therefore

dr'

'^'—^Jf&^''^^-

The expression is left in the first instance in this shape in
order to show more clearly tlie manner in which each portion of
the disturbance in the state taken as initial contributes towards
the future disturbance at F. When there is no obstacle to the

transmission we shall have j dcp—^ir, and / ( -7-7)^^^''= (^o)

taken between limits. If It <PQ, the sphere round P with
radius M does not cut the disturbed region at all, and the
disturbance at P is nil. If bt^PS, the limits of / are the
distances from 0 at which the sphere round P cuts the inner
and outer limits of the shell, and as the disturbance there van-
ishes we have again no disturbance at P. But if M lies be-
tween those limits, and the sphere round P cuts OP in T
(which point must lie between Q and 8) the limits of r' will be
OT io a point in the outer boundary of the shell, where there-
fore ^0 vanishes. Hence the displacement at P is the same as
was initially at T, only diminished in the ratio of r-\-r' to '/,
as we know it ought to be.

Reverting to the expression for I given by the double inte-
gral, we see that the only portion of the shell which is efficient
in producing a subsequent disturbance at P lies between the
sphere round 0 with radius OQ and the sphere round P with
radius Pas'. If /3 be the distance from OP of the intersection
of these spheres, we have, considering the smallness of the ob-
liquities.

If we suppose r and r' to be each 4 inches, and \ the mill-
ionth of an inch, we have /3 = 0.002 inch, so that at a distance
not less than the one-250th of an inch from the projection of
E 65

MEMOIRS ON RGNTGEX RAYS

the edge of an opaque body intercepting Eontgen rays coming
from a point 4 inches off, and received on a screen (fluorescent
or photographic) 4 inches on the other side, there would be
full effect or no effect according as we take the illuminated or
the dark side of the projection. We see then how possible it
may be to have an almost complete absence of diffraction of
the Rontgen rays if the pulses are as thin as above supposed ;
and as these rays are started in the first instance in a totally
different manner from rays of ordinary light, namely, by the
arrival of charged molecules from a cathode at a target instead
of by the vibrations of the molecules of ponderable matter, we
know of no reason beforehand forbidding us to attribute an
excessive thinness to the pulses which the charged molecules
excite in the ether.

Biographical Sketch

Sir George Gabriel Stokes was born August 13, 1819, in
Ireland, County Sligo, and is at the present time Fellow of
Pembroke College and Lucasian Professor of Mathematics in
the University of Cambridge. He was Senior Wrangler in
1841 ; he has been President of the Royal Society of London,
and has received numerous honors at home and abroad. His
main contributions to science may be grouped under the head
of Hydrodynamics and the Wave Theory of Light. His papers
on the former subject gave the first rigid treatment and formed
the basis of our modern theory. Similarly, in the wave theory
of light his papers on the dynamical theory of diffraction and
on the aberration of light are two of the most important con-
tributions of modern times to science. His collected papers
are being published at the present time by the University of
Cambridge, and two volumes have already appeared.

His experimental work has been largely in connection with
such optical phenomena as fluorescence, metallic reflection,
and certain anomalous colors seen in crystals.

His mathematical work is of the first importance, and his
numerous contributions to all branches of mathematical physics
have been of the greatest service to science.

A THEORY OF THE CONNECTION BE-
TWEEN CATHODE AND RONTGEN RAYS

J. J. THOMSON, M.A., F.R.S.,

Cavendish Professor of Experimental Phj'sics, Cambridge
(^Philosophical Magazine, February, 1898)

CONTEXTS

PAGE

ElietrijSed Bxrtide JM&tiiuf in Magrndie Fidd, 69

Mntffen, Eitjf*, thin PnU^ 70

Veltmty qf Mmtiffen and Lmard Mxiftt. 71

Pul»^ Gon^iM <sf Eleetrk and MMgntftic Di^uHkinoe* 71

Efeet of Time «f ChUintfn, 71

0»mmunimti*!fn of Energy to Charsf&i lom 73

DiMwrianee Greated at Right Angk* to the Cathode Ba§« 72

Mntgen Bajf* mot Wanes, but Imput*e*. 73

A THEORY OF THE COXXECTIOX BE-
TWEEN CATHODE AXD ROXTGEX RAYS

J. J. THOMSO\,.M.A., F.R.S.

A MOTIXG electrified particle is surronnded by a magnetic
field, the lines of magnetic force being circles having the line
of motion of the particle for axis. If the particle be sud-
denly stopped, there will, in consequence of electro-magnetic
induction, be no instantaneous chancre in the ma^etic field ;
the induction gives rise to a magnetic field which for a mo-
ment compensates for that destroyed by the stopping of the
particle. The new field thus introduced is not, however, in
equilibrium, but moves off through the dielectric as an electric
pulse. In this paper we calculate the magnetic force and
electric intensity carried by the pulse to any point in the di-
electric.

The distribution of magnetic force and electric intensity
around the moving particle depends greatly on the velocity
of the particle : if this velocity is so smaU that the square
of its ratio to the velocity of light can be neglected,
then the electric intensity is symmetrically distributed round
the particle, and at a distance r from it is equal to ejr^,
where e is the charge on the particle ; the lines of magnetic
force are circles with the line of motion of the particle for
axis : the magnitude of the magnetic force at a point P is
we sin Q/r^, where ic is the velocity of the particle, and 0 the
angle a radius from the particle to P makes with the direc-
tion of motion.

When, however, the velocity of the particle is so great that

MEMOIRS ON

we can no longer neglect the square of its ratio to the velocity
of light, the distribution of electric intensity is no longer uni-
form ; the electric intensity, along with the magnetic force,
tends to concentrate in the equatorial plane — that is, the plane
through the centre of the particle at right angles to its direc-
tion of motion ; this tendency increases with the velocity of
the particle until, when this is equal to the velocity of light,
both the magnetic force and the electric intensity vanish at all
parts of the field, except the equatorial plane, and in this plane
they are infinite.

The pulses started by the stopping of the charged particle
are, as might be expected, different when the ratio of the ve-
locity of the particle to that of light is small and when it is
nearly unity. But even when the velocity is small, the pulse,
started by stopping the j^article, carries to an external point a
disturbance in which the magnetic force is enormously greater
than it was at the same point before the particle was stopped.
The time the pulse takes to pass over apoint P is, if the charged
particle be spherical, equal to the time light takes to pass over
a distance equal to the diameter of this sphere ; the thickness
of this pulse is excessively small compared with the wave-length
of visible light. When the velocity of the particle approaches
that of light, two pulses are started when it is stopped. One of
these is a thin plane sheet whose thickness is equal to the di-
ameter of the charged particle ; this wave is propagated in the
direction in which the particle was moving ; there is no corre-
sponding wave propagated backwards : the other is a spherical
pulse, spreading outward in all directions, whose thickness is
again equal to the diameter of the charged particle, and thus,
if the particle is of molecular dimensions, or, perhaps, even
smaller, very small compared with the wave-length of ordinary
light. The theory I wish to put forward is that the Rontgen
rays are these thin pulses of electric and magnetic disturbances
which are started when the small negatively charged particles
which constitute the cathode rays are stopped.

[^The matliematical theory is omitted.^

Thus we see that the stoppage of a charged particle will give

ROXTGEX RAYS

rise to very thin pulses of intense magnetic force and electric
intensity ; when the velocity of the particle is small there will
be one spherical pnlse ; when the velocity is nearly equal to
that of light there will, in addition to the spherical pulse, be a
plane one, propagated only in the direction in which the par-
ticle was originally moving. It is these pulses which I believe
constitute the Rontgen rays. As they consist of electric and
magnetic disturbances, they might be expected to produce some
effects analogous to those of light. If they were so thin that
the time taken by them to pass over a molecule of a substance
were small compared with the time of vibration of the mole-
cule, there would be no refraction, and the thinness of the pulse
would also account for the absence of diffraction.

In the preceding investigation we have supposed that the
stoppage of the particle is instantaneous ; if the impact lasts for
a finite time T, the negative pulse will be broadened out, so
that its thickness, instead of being 2a, will be 2a-\-\T, where
V is the velocity of light. The intensity of the magnetic
force in the pulse will vary inversely as the thickness of the
pulse, so that, when the collision lasts for the time T, the mag-
netic force in the negative pulse will be 2a}{2a-\-\T) of the
value given above. The more sudden the collision, the thinner
the pulse and the greater the magnetic force and the- energy in
the pulse ; the pulse will, however, possess the properties of
the Rontgen rays until T is comparable to one of the times of
vibration of a substance through which it has to pass. In the
case of the cathode rays all the circumstances seem favorable
to a very sudden collision, as the mass of the moving particles
is very small and their velocity exceedingly great. In some ex-
periments which I described in the Philosophical Ilagazine for
October, 1897, on cathode rays, the velocity of the negative
particles was about one-third of that of light, and in some more
recent experiments made on the Lenard rays, with the appa-
ratus described by Des Coudres, considerably higher velocities
were found. A change in the time of the collision will alter
the thickness of the pulse, and so change the nature of the
ray.

MEMOIRS OX

If we suppose that part of the absorption of the rays is clue
to the communication of energy to charged ions in their path,
we find that the thicker the pulse the greater the absorption.
For, suppose that E is the electric intensity in the pulse, m the
mass, and e the charge on an ion ; then, if u is the velocity com-
municated to the ion when the pulse passes over it, t the time
taken by the pulse to pass over it,

7nu='Ee . t ;
or, if cl is the thickness of the pulse,

thus the energy -imc^ communicated to the ion is equal to

1 E\ri

2 \'

Xow the energy in the pulse is proportional to E'^ d/Y^, so
that the ratio of the energy communicated to the ion to the
energy in the pulse is proportional to d. Thus, the broader
the pulse, the greater the absorption and the less the penetrat-
ing power. The energy in the pulse is inversely proportional
to its thickness.

If we return to the expression for the intensity of the mag-
netic force in case (1), we see that it is proportional to sin 0,
so that the disturbance is greatest at right angles to the
cathode rays : thus, if the cathode particles are stopped at their
first encounter, the Rontgen rays would be brightest at right
angles to the cathode rays ; if, however, as would seem most
probable, the cathode particles had to make several encounters
before they were reduced to rest, changing their direction be-
tween each encounter, the distribution of the cathode rays
would be much more uniform. Experiments on the distribu-
tion of Rontgen rays produced by the impact of the cathode
particles directly against the walls of the discharge-tube are, as
Sir George Stokes has pointed out, affected by the much greater
absorption of the oblique rays produced by the greater thick-
ness of glass traversed by them. Experiments on rays produced
by focus-tubes would give results more easily interpreted.

RONTGEX RAYS

The result to which we have been led from the consideration
of the effects produced by the sudden stoppage of an electrified
particle, viz., that the Rontgen effects are produced by a very
thin pulse of intense electro-magnetic disturbance, is in agree-
ment with the view expressed by Sir George Stokes in the
Wilde Lecture (^^Proceedings of the Manchester Literary and
Philosophical Society/' 1897), that the Rontgen rays are not
waves of very short wave-length, but impulses.

Cambridse, December 16, 1897.

Biographical Sketch

Joseph Johx Thomson was born Dec. 18, 1856, in Manches-
ter, and is at the present time Fellow of Trinity College and
the Cavendish Professor of Experimental Physics at Cambridge.
Professor Thomson was second Wrangler in 1880, and was ap-
pointed to his professorship in 1881:. He has contributed
greatly to our knowledge of both practical and theoretical
physics ; his researches on the theory of vortices, and on the
application of dynamics to physical problems, have been pub-
lished in book form. The greatest debt that science owes
him, however, is for having introduced system and order into
the vast collection of experimental data which have been ac-
cumulated concerning the discharge of electricity through
gases. His work on this subject has been carried on during
the past ten years, and the most important conclusions are con-
tained in his volume Recent Researches in Electricity and Mag-
netism, Oxford, 1893. Professor Thomson has contributed
largely to our present knowledge of cathode rays and to all
that pertains to the connection between matter and electricity,
particularly to the explanation of electrolysis and ionization.

BIBLIOGRAPHY

Among the most important papers bearing upon the subject of X-ra}?;
are the following :

Cathode Rays :

Lenard, Wiedemann, Annalen, 51, 1894 ; 56, 1895 ; 63, 1897.
Thomson, J. J., Philosophical Magazine, 38, 1894 ; 44, 1897.

British Association Report, 1896.
McClelland, Proceedings Royal Society of London, 61, 1897.
Perriu, Comptes Rendus, 121, 1895.

RoxTGEN Rays :

Thomson, J. J., Proceedings Royal Society of London, 59, 1896.

Nature, 53, 54, ^5, 58, 1896-1898.
Perrin, Annales de Chimie et de Physique, 11, 1897.

Comptes Rendus, 122, 123, 124, 126, 1896-1898.
Murray, J. R. E., Proceedings Royal Society of London, 59, 1896.
Wilson, C. T. R., Proceedings Royal Society of London, 59, 1896.
Lehmann, Zeitschrift fUr Electrochemie, 1, 1896.
Lord Rayleigh, Nature, 57, 1898.
Les Rayons Xet la Photographic a Travel's les Corps Opaques. Par Cli.

Ed. Guillaume, Paris, 1896.
Rontgen Rays and Phenomena of the Anode and Cathode. By E. P.
Thompson, New York, 1896.

Becquerel Rays :

Becquercl, H., Comptes Rendus, 122, 123, 124, 1896-1897.
Sagnac, G., Journal de Physique, 5, 1896.

A Resume of the Experiments dealing with the Properties of Becquerel
Rays. By O. M. Stewart, Physical Review, 6, April, 1898.

Thorium Rays :

Schmidt, G. C, Wiedemann, Annalen, 65, 1898.
Sklodowska-Curie, Comptes Rendus, 126,1898.

INDEX

Absorption of Rays bv Charged
Ions, 72.

Air, Active when Exposed to X-rays,
15, 21, 22 ; How it Loses Activi'ty,
16 ; ^lore Transparent to X-rays
than to Cathode Rays, 10.

Aluminium, Transparency of, 4. 1-3,
27, 30, 52.

Barium Platino- Cyanide, 6, 23;

Screen of. 3, 21, 22. 23.
Becquerel Rays, 48, 62, 63.
Bibliography, 74.
Blackness, Cause of, 61.
Brandes's Experiment, 39, 40.
Brightness, Conditions of, 24.

Calcium Tungstate, 23.

Cathode Ravs, 10. 11. 24. 35, 38, 39,
70, 72 ; Speed of, 71 ; Relation to
Rontgen Ravs, 70.

Cathodic Rays, 43, 44, 49, 50, 52,
53 ; Stream's of Electrified Mole-
cules, 53.

Crookes, W., Experiments of, 50.

Current, Effective, in Tubes, 34.

Density, as Determining Transpa-
rency. 5.

Diffraction Phenomena. 40, 46, 53,
55, 56, 57, 63, 64, 65, 71.

Direction, Variation of Intensity
with, 24.

Edison Screen, 23.

Electrical Intensity and Magnetic
Force, Carried by Pulse, 69."

Electrified Bodies, Discharged by X-
rays, 13.

Energy in Pulse, Inversely Propor-
tional to Thickness, 72.

Eye, Sensitiveness to X-rays, 7, 39.

Fluorescence, Production of, by X-
rays, 3, 6, 36, 59.

Galitzin, Prince, 44, 46, 47. 60.
Glass. Transparencv of, to X-rays,

4, 17.
Green, George, 48.

H
Hertz, Heinrich, 10, 35.

I
Interference, 12.
Interrupters of Deprez and Foucault,

31, 33.
Ions, Absorption of Rays by Charged,

Karnojitzky, von, M.. 44, 46, 47, 60.
Kelvin. Lord, on Static Charge, 53.
Kinetic Energy of Eiher, 61.

Lambert's Law, 25.

Lenard Ra\'s, Nature of. 52 : Speed

of, 71.
Lenard "s Experiments, 10, 36, 49, 50,

51. 52.
Longitudinal Vibrations in the Ether,

13, 47.

INDEX

M
Magnetic Force, Carried by Pulse,

69.
McClelland, C. M., 45.
Molecules, Statically Charged, 53.

N
Normal Rays, More Copiously Emit-
ted, 45.

Photographic Action, 6 ; and Fluo-
rescent Action Different, 37.

Photometer for X rays, 23 ; Weber,
10; Bouguer, 23.

Platinum- Aluminium Window, 29 ;
Transparency of, to X-rays. 4, 17.

Polarization of X-rays not Proved,
47.

Powders, Effect of, 8.

Pulses, 53, 54, 72 ; by Impact of
Charged Particles, 69, 70, 71 ;
Vast System of, Mathematically
Treated, 63, 65 ; Velocity of, 70.

Radiation, Intensity of, 23, 24.

" Raj''s" Defined, 11.

Reflection not Observed with X-
rays, 8, 46.

Refraction, Nature of, 58, 59 ; New
Theory of, 58, 63 ; Not Observed
with X-rays, 7, 46, 71.

Rontgen, W. L., Biographical Notice
of, 40.

Rontgen Rays, a Vast Succession of
Independent Pulses, 54; Difference
of Quality in, 56, 57 ; Thin Pulses,
Electric and Magnetic Disturb-
ances, 70.

Shadows, Sharpness of, 11, 44, 46,
57 ; Double, 44.

Spark -Gap. 32.

Stokes, G. G., 26, 43, 73 ; Biographi-
cal Notice of, 66.

Tesla Apparatus, 17, 13, 32.

Thickness, P2ffect of, on Transpa-
rency, 5; Relation of, to Density, 6.

Thomson, J. J., 45 ; Biographical
Notice of, 73.

Transparency of Different Bodies,
4, 26, 27, 46.

Tubes, Hard and Soft, 30, 33 ; Hit-
torf, Lenard, Crookes, 3, 9, 10,
35, 39, 50 ; Variation of, with
Use, 33.

Velocity, Dependence of Pulse on,

70.
Vibration, Longitudinal, in the

Ether, 13, 47.

Weber Photometer, 10.
Wiedemann, E., 52.
Wilde Lecture, 43.
Window-Number, 29.

X-rays, Best Tubes for, 24 ; Chemi-
cal Action of, 6 ; Difference with
Hard and Sofi Tul)e«;, 31 ; Diffrac-
tion of, 40 ; Discharge Electrified
Bodies, 13 ; Discovery of. 3 ; Dis-
tinction from Cathode Rays, 11 ;
Effect of Current on, 34 ; Intensity
of, 23 ; Interference of, 12 : Mixt-
ure of, 35 ; Named, 4 ; Nature
of, 12, 43 ; Not Deflected by Mag-
net, 10 ; Origin in Tube, 11. 38,
43 ; Photograph by jNIeans of. 6 ;
Photometer, 23 ; JPlatinum Best
Suited to, 18 ; Propagation in the
Ether, 46 ; Rectilinear Propaga-
tion of, 11, 12 ; Succession of
Pulses, 54 ; Transparency to. 3,
4, 10, 26. 27, 28; Transverse Vi-
bration of, 48.

Zehnder Tubes, 34.

THE END

STANDARDS IN NATURAL SCIENCE

COMPARATIVE ZOOLOGY

Structural and Systematic. For use in Schools and Col-
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